(rest of your post is good too, just highlighting this one).
I get so pissed off when people ask for GOG to make a "Steam Workshop" equivalent. It's like, no... Steam Workshop was a giant step backwards for game modding! It's a huge "lock in" tool that *has* prevented titles from being released outside of Steam.
I'm suspicious of this. Does the person game on an a computer that is kept offline entirely? Is it possible he's online and doesn't realize it? If the reports are fully accurate, he really is content with a game with but one weapon and a just couple stages?
Bro just trust him!!
While Steam Workshop is good for those who own the game on Steam and an easy way to download lots of mods for lots of different games over there - yeah, that's really it.
It doesn't solve the problem of say if you bought the game on say GOG, as they don't have a Workshop; you'd have to go hunting for the mod & find it...if you can.
You might need to find the actual files all over the Net, otherwise - like on a site if you can (like PC Gaming Wiki, if a mod is actually there) or say check Nexus Mods to see if it's over there.
Hopefully, file-types (like ZIP or whatever) are the same for SteamWorkshop and NexusMods for usage - as it would make it easy for those modders to throw them up on both sites. Thing is: will every modder throw their mods up on Nexus, even if it's already up on Steam WorkShop?
But, again - any proprietary stuff....yeah, when a game's going from one store to another - say the dev's likely have to say remove Steam Workshop support when they are re-porting that version over to the GOG version. That's going to take time, money, resources, man-power - so, that means they basically have to make another version just for GOG...and to support GOG's Features. For example, Shadowrun Trilogy has WorkShop Support on Steam - but there's no workshop over on say GOG...so I'd guess users/modders would need to surf online and/or check Nexus Mods for some modules for those games.
What's probably nice about really old games for dev's probably (i.e. from before Steam came out) - there was no achievements, no stat boards, no leader boards, and nothing else - so it's probably easy just for dev's to just toss the game onto Steam and GOG.