Couuld it be possible to release the supporter pack here, on GOG?

I'm interested in this as well. I'd prefer to buy the game on GOG but I'm also curious why that extra isn't available here,

Well. I searched everywhere for 'Skirmish Mode Games' but I don't think they are a thing anymore.
The developers now, or at least one of them, is at workingpixel.com so I sent them an email.
Doubt they care about this game anymore, seem to have several projects.
I got the game on epic, and epic has the supporter pack so I'll probably grab it there as I don't spend money on gog anymore anyways.

Edit: Well, this game is a complete mess. Not the game itself, but the lame developers and nonexistent publishers.
The current version on steam is 2.0.16. Epic is 2.0.8, and I think gog is 2.0.5.
Gog has no supporter pack.
Epic has supporter pack but it doesn't work. First time buying a thing like that on egs, and it's extremely stupid. You have to download all of the 'dlc' into a folder. So if you just wanted the soundtrack, you can't download the soundtrack, you have to download the entire dlc. No instructions where you're supposed to put any of these random files, but it doesn't allow you to download to the game directory, and customer support is worthless everywhere you go now a days.

Pretty odd, this game is actually a ton of fun.