I am currently working on a "smart" collection of keys that can be used by GMK GB designers to add a decent ISO-International support to their GB without much trouble and effort.
Since you already included most of the ISO-UK keycaps in your set, there are "only" 25 additional keys required to add very solid support for UK, Germany, Sweden, Norway and Denmark while also providing much improved but still only partial support for Spain, Italy and Portugal. Since ISO-International support has never been done before in a GMK GB, this would make any GMK GB with ISO-International support the more appealing to us ISO folks.
Here's the current version of effort, the _GREEN_ keys are the ones that I am advertising here!
Here's a link to the KLE work-in-progress project for your reference:
http://www.keyboard-layout-editor.com/#/gists/6f7830fa68c43c022e3d691bddb1a9a2If you click on the layout name you can see the legends.
Depending on the MOQ we reach with this kit, I believe the markup for adding these keys should be <20$ even with just 150 sets, getting closer to 15$ the higher the MOQ gets. Still waiting for a statement from GMK but that's what I would expect with the experience from my recent GMK GB.
EDIT: So yeah while I'm well aware the chance these keys are considered for the current GB are slim I would still love to get some feedback from Brazzers. She's been through a lot of input in this GB and this is a very lovely and popular IC with the potential to reach high MOQ. Would be very interesting for me to hear her opinion about internationalizing a GMK GB.
EDIT2: Got a response from GMK regarding the potential markup when adding these keys to an exiting 100+ keys base kit and even with a 150MOQ the markup should be less than 20$!