It is always a really lazy and bad decision if developers make mod support exclusive to Steam Workshop.

Take a look at "The Riftbreaker". That game is made by quite a small company, but has full mod support on all platforms.
Even more, it has integrated mod support into the game which is not tied to Steam.
If those people can get it done, others could, too.

Isn't there a website, which can download all Steam workshop mods?
There is a site that can get mods from Steam Workshop, but it's not able to download the mods for that game.
The only site that most probably could have been able to obtain them received a "cease and desist" letter from VALVE, and closed many months ago...

As of now, the ONLY way to use the Steam Workshop MODs of "Starship Troopers: Terran Command" on the GOG version of the game is this:

1) Buy the game on Steam
2) Subscribe to the mods you want
3) Start the game on Steam (this is the only way to trigger the download of the MODs)
4) Get the downloaded MODs from the game folder (of the Steam version)
5) Put the MODs in the game folder of the GOG version
FREE STUFF (for limited time)
It's interesting that when I search something about "GOG" on Google, it proposes these phrases as the most common used on the argument:

Dying Light - Definitive Edition just received a 500 MB Patch without changelog... I suppose it could have something to do with the Epic Store version, since it will be a Free game there the next week... anyway, kudos to Techlan for continuing supporting a (superb) game even if they already have released it's sequel.
Also, about the GOG version in particular, in the past they also included crossplay and all the stuff present on the other stores, and found also the way to give things that were "exclusive" of some site available for all versions thanks to the live events.

On a side note, the latest patch for GOG GALAXY was one of the best of the last years... this doesn't really mean that it's awesome, but for once it included a lot of fixes for things that were receiving a lot of complaints for many months... so, this was somehow how every patch should have been.
FREE STUFF (for limited time)

(for some reason the plugin doesn't work on the URL...)