I've owned keyboards from all of those brands, so I might be able to help.
1. Varmilo probably has the best quality in everything but the PCB. I don't know how that compares to the Ducky (which is next in quality).
Varmilo has dye-sub PBT keycaps in SS2 profile, which is very good for typing. [And gaming, which can be less discriminating].
Varmilo cases usually look and feel relatively luxurious.
Varmilo has limited lighting options, in that most of them use single-colour LEDs.
Most important, they tend to have good stabilizers out-of-the-box, which a lot of keyboards don't. They may even be lubed. The keys feel firm and well-seated, and it doesn't rattle as much as other boards.
Ducky is good-for-the-price. They actually have double-shot PBT keycaps, I think, which is even better than dye-sub. But they are OEM profile, which people here don't prefer as much. As a beginner, you may appreciate OEM more, since it is more common.
Stabilizers are not as good as Varmilo, I find. They aren't bad, but they aren't great either. The case is also pretty mundane, although solid.
You can get Ducky models with RGB, or single-colour LEDs.
My Ducky One has held up for years, and they are known to be reliable.
The GMMK is the least 'solid-feeling' board. It's almost like a plastic switch tester with RGB. That doesn't mean it's a bad board. I haven't seen any reliability issues with it yet. But it feels plasticy for the most part, and they used to have horrible stabilizers. This has been fixed somewhat in recent years, but it's not like it comes with awesome stabilizers. Also, the keycaps it comes with are very mediocre, at best. It's a hollow board.
It's main strength is hot-swapping. It's really a different kind of board, for different purposes [testing different switches].
It has good RGB for the price, and yes, you can customize it. More of a 'gamery' board.
Varmilo = most luxurious, more expensive, more 'enthusiast' oriented (e.g. people who like customs). Some QC problems with PCBs.
Ducky One = solid, reliable, good-for-the-price, should be hassle-free. Missing some luxuries.
GMMK = switch tester with lights, good customizability but limited design.
2. I have a Varmilo that has white backlit keys, and it is pretty good. It looks white, instead of some weird blue colour. But I've heard different things from other people. Mine was a factory-custom, but the ones you can buy off the shelf might have off-white lighting. You can check the reviews on "Drop" for more information.
The thing about the Varmilo backlight is that it is a general, ambient backlight. It's meant to seep through the keys, rather than illuminating the letters.
This is because Varmilo's are marketed slightly more towards people who like non-backlit keys that look good in ordinary, ambient lighting. So it's about the look of the keys, rather than the backlighting. The Varmilo LED lighting is mainly there to accentuate the keys.
3. I have heard some bad things recently about the latest Ducky Shines, that they are not so much an improvement over the previous generation, and maybe a downgrade. I don't know if that applies to spacebars, but I would avoid the latest Ducky Shines.
I haven't heard the same thing about the One 2. It generally is seen as a good, solid keyboard for the price. I kinda wish I had waited for the One 2, as I could have bought one with Skyline or Horizon, or even Good in Blue.
4. The Ducky software isn't that great, in my opinion. I had difficulty in finding out which was the correct software to install for my particular model [Ducky One]. They didn't hold my hand. I don't think I ever got it operational.
Maybe things have changed since then.
The GMMK was easy to install and setup, and fun. But it's also somewhat limited. But good enough. Just make sure you're not using it on a small computer, as it takes up more than the 1377x768 screen on a netbook.
Varmilo gives you no control, as I understand it, since most of their keyboards use single-colour LEDs. So you better pick a pattern than you like, at the custom shop. [The off-the-shelf ones tend to use all-white, which is fine, as it is neutral and versatile. But watch out for that bluish tint on some models.]
5. If I were buying it for myself, I would buy the Varmilo, although I don't like the CMYK colour pattern and would probably choose a different model. [They have all sorts of crazy ones, including one that comes with a stuffed panda.]
The Varmilo feels really nice, like they really cared when they made it.
But the Ducky is also good. I would get the One 2 RGB if you haven't heard of any QC issues and it's reasonably-priced. Because the base One 2 is good, and I also wish I had one.
The Varmilo and One 2 should be solid buys. [I would avoid the Shine 7 for now].
The GMMK is also something you should pick up later. What you should do is wait to the Black Friday or holiday sales. Because they get rid of their excess keyboards for cheap.
During the sale, pick up either a TKL or full-size, whichever is cheapest. The current TKL has a detachable cable, but the old full-size didn't. So get whatever you can get the best deal on.
Interestingly, the small GMMK, the 60% is said to have the best stabilizers of all, so you might want to try that.
The reason I say get the GMMK on discount is that you may in the future want an even fancier keyboard with custom switches. But you won't know which switch to use. But you can experiment for cheap on the GMMK. You don't even need a big one, in fact it's cheaper and easier to outfit the smallest GMMK with a few bags of switches.
The thing about Varmilo is, I think that their keyboards are honestly the best that Varmilo can churn out at a profit, even though some of their technology is outdated compared to competitors [e.g. single-colour LEDs, and questionable PCBs]. So it's like the very best they can put out with their mostly but not completely up-to-date equipment. And it shows.
The Ducky is made to a lower price point, and it also shows, in my opinion. But they have doubleshot PBT keycaps which should go well with the Silent Red switches [I have non-silenced reds with mine]. So it should be entirely adequate for your purposes, without spending more than you need to.
The GMMK is for slowly and cheaply testing switches over time, once you can get one cheaply.
TL;DR:
Varmilo and Ducky One 2 should be solid buys.
Get a GMMK when they go on major [like 30% or more] sale. I got mine at least 40% off.
