There is two things to to watch out for on it, the first is the cable, get a magnetic USB cable it will protect the port which is quite fragile on the GMMK, it's a micro USB that just hangs out the back. You can get these for a few bucks, worth every penny.
Is this the right kind of magnetic micro-USB? I've never heard of these so I was unsure of what to look for, but the only other keyboard cables I've seen are the tightly-coiled ones that are shown frequently on r/MechanicalKeyboards.
I have never used magnetic cables myself, but probably will get some for my Oculus Quest at some point, if nothing else. The only thing I would add is to be careful which ones you do get. Some do not support data transfer and are merely for charging. These appear to do both, and you've got Jerry vouching for them.
Firstly you need a lot more LEDs to light up the top of the board evenly, more or less one LED per switch, and therefore it's more expensive to do that than put underglow on (which is often only a dozen LEDs) and secondly the most popular after market keycaps are solid, rather than shine-through, so you get a better visual effect from a case with an acrylic bottom and some underglow than shining up. Regardless of the reasons, it's also much harder to add RGB backlight to a keyboard than underglow after the fact. Most PCBs only come with holes for putting single-colour LEDs.
The more I fall into this rabbit hole the more I'm starting to question if I really MUST have per-key RGB. I'm so used to having it because of the retail boards I've used. Since I'm creative as well as tryhard-organized, it was satisfying to have custom lighting presets for each game I play that "matched" the in-game HUD and color schemes. However, I did notice that most keycap sets are entirely solid and that as cool as the pudding caps look, there are so many more options out there I could try. I feel really undecided on this for the moment, but I appreciate the information regardless!
Others would know better than I, since I don't normally do the programmable and/or kit keyboard thing myself, but I don't know that you would have very many (if any) options for per-game RGB settings outside of the gaming board market. I have used my K70 since before that stuff even existed (to my knowledge), so I guess I have gotten by just fine without it. lol
You'll usually find more custom 96% boards than full size ones (at least in DIY kits) because it's easier to make a keyboard case and plate where everything is together than having to machine a case to seperate out the arrow cluster, navigation cluster, and numpad into separate blocks. One of the only ones I can think of off the top of my head is the KBD19X, but it's more expensive and only intermittently available. Still cheaper than a full custom though.
I actually would love a keyboard with that sort of layout! I have small hands but I also am so used to a full keyboard that seeing everything on top of one another bothered me to look at. That 96% with the very slight spacing would be a great balance I think, so it's a shame that they're a bit harder to find. Perhaps I could get used to the more compact design in time, especially with having the numpad so much closer to my hands' resting position, but I'm hesitant to invest too much into it without being able to see it in front of me if that makes sense? 
I routinely use a full-sized keyboard when gaming ... not because I particularly wanted that, but because I wanted that board and that's what it was. There are a lot of people who find merit in having a shorter keyboard to facilitate having your hands closer together when gaming. I'm not sure it makes any difference for me in limited testing, but that may be because of relatively broad shoulders. if you must have a number pad, and think that having your hands closer together may be a benefit, I could see that layout being a good contender.
Are you talking about the GMMK in the quote you're commenting on? Those boards look to have the switches installed already. The difference between 3 and 5 pin is what Leslieann was touching on. Some switches are meant to be plate-mounted (3 pin), and some are meant to be PCB-mounted (5 pin)... The extra nubs on either side are what you would clip off for a PCB that doesn't support them.
I was under the impression that if I ordered a custom GMMK instead of just buying the stock one that there would be "some assembly required". If that's my misunderstanding and I don't need to install the switches either way, then that's great!
The description for that keyboard says the following:
The full sized, RGB, modular mechanical keyboard. Comes preinstalled with Gateron brown switches & black keycaps. Ready to go - requires no setup. Perfect for someone new to mechanical keyboards.
I don't buy a lot of ... enthusiast things, unless they're 25+ years old, but that reads to me like it is a hot swappable board but comes with the switches preinstalled.
I'm not sure about paint pens, but permanent marker is less permanent than most people think. It will wear off faster than you expect.
Ordinarily yes, but I'm a multimedia artist and have a lot of different sealants and varnishes that I can use to protect the ink from wearing off on my fingers once it dries.
Cool. I was just making sure. I have some caps I dyed, some people having claimed that it seeps beneath the surface of the plastic and does not wear off ... and it started wearing off in a week. I'm using a ton of hand sanitizer, so I imagine that doesn't help, but still. If you do embark on an adventure like that, it might be interesting/informative to post in the
Keyboard Keycaps section. Maybe with some long-term durability updates.
This is an opinion, but a widely-held one: There are way better clicky switches than anything you're ever going to feel in a Best Buy. There are 3 main categories of switches: clicky (also tactile), tactile (tactile but not clicky, generally more quiet and not as crisp), and linear (like your silent reds). If you're anything like me, once you've had a chance to try good clickies, you won't ever want to go back to MX or clones (like Gateron) again.
If I reference the guide on the GMMK page just for what they offer, it suggests Gateron Blue, Gateron Green, or Kalih Box White. I'm not sure what else is similar but I can't really invest in a switch-tester for the time-being if I also want to get a new keyboard before the holidays. I think for now I'm going to stick with the Cherry/Gateron Blue because I know that I do enjoy them at least compared to what else I've tried. Once I can seriously start the process of making myself a full custom I will get a switch tester to ensure I get exactly what I want from the huge selection available. I have many ambitiously creative ideas for my custom keyboard that may or may not be possible to even create, but I'll figure that out when the time comes. 
You may prefer the Gateron blues, between those three. The Gateron green will be very heavy if you're used to MX blue. Gateron blue is probably the best MX or clone clicky switch, followed closely by Outemu (that may be debated). Both the Gateron and Outemu blues are a bit more tactile, and definitely more consistent than MX. Box whites are an entirely different click mechanism, they use a click bar, like the jades I recommended. The box white click bar is very thin though ... I'm not sure why OEMs are pushing those when most people like pale blues, jades, pinks, and navies. Box click bar switches are very smooth and crisp, not at all crunchy/rattly and inconsistent in feel and sound like MX. Box whites are a very pleasant switch, but less tactile than you would be used to.
Fair warning: The clicky journey usually leads to 80s keyboards.
Good luck on your path to keyboard nirvana
I'm not sure where you're looking. These are currently out of stock, but this is the going rate on NovelKeys. That's $3.50 per pack of 10, so 100 switches would be $35... I just don't care enough about trying the switches to bother to buy loose ones to solder into a board I already have, and definitely don't care to desolder them from that board unless I decide they're the best thing since sliced bread.
That's my bad then, since I only really used Google to get some search results for places that did have the reds in stock. When you put it that way I understand why it'd be a lot easier to just test mine for a couple days before you invest a lot of time and effort into putting them in your own keyboard. I will be continuing to update this thread as I go through this process and find my new board, and will let you know once I have it in case you are still interested.
Unless I find another cheap silent red board before then, or pull the old return switcheroo with Best Buy, I'll still be interested. I got the $100 board on Ebay at a steep discount, but it turned out the description was wrong and it had regular MX reds. That offer, of course, was only if you end up otherwise trashing the thing, etc. I'm in no hurry.
You're welcome. Soldering switches is some of the easiest soldering you'll ever do, you may even find it therapeutic if you ever want to give it a try. Desoldering can be a pain, especially without some pretty expensive tools, and even then it can still take hours for a whole board.
Ever since I got into refurbishing and customizing old Gameboy systems, I've been slightly interested in learning basic soldering. It's possible to add backlit screen mods to the older systems but it requires one soldering spot and I was too nervous at the time to try it. I do have an iron now and a few friends & family with experience, so I'll definitely be learning it eventually since I'm good with having steady hands and performing small detail work.
I have always thought about doing a backlight mod to a GBC ... I hesitate to modify the GB I played Pokemon Red on though. I still have my atomic purple GBC from the 90s. I have a Gameboy Pocket that needs some sort of screen repair. Do you happen to know what's most common to go wrong on that model? When I picked it up for almost nothing from a Goodwill, I didn't have the required proprietary tri wing screwdriver bits. I have a Sega Game Gear that probably needs a little attention too.
I'm surprised you've pulled off doing any refurbishment or modification without a soldering iron, but if the 3DS, etc, is any indication ... Nintendo does love a billion tiny fiddly little ribbon connectors, etc.
My hands are shaky as hell sometimes and draw some pretty terrible stick figures, I still get by just fine with some pretty precise soldering.
Tips like this help a lot, if you do ever end up doing some soldering on some really tiny/close solder pads. The general tips are fine for most things in a keyboard, and I imagine an old Gameboy, but they're useless for anything relatively small ... like the micro USB solder pads I was trying to solder to this past weekend. Cheap irons usually don't have many options for tips either.
The important thing is to prevent from overheating the circuit. Better irons/stations have temperature controls, and either way you just don't want to hold the iron on anything for longer than you need to for a good clean connection. Besides that, the only thing that really matters is not jerking too hard on pads, etc. If you avoid both of those things, you can usually play around with an iron all you want without much problem.
There's no shortage of random old electronics nobody wants or cares about to practice with.
