For a game like RimWorld, which has extensive mod support on Steam - how is this covered on GOG ?

Try the Nexus

https://www.nexusmods.com/rimworld

Also Mod DB:
https://www.moddb.com/games/rimworld/downloads

I have linked to the Rimworld seciton of both of these, but they have tons of other games that are on gog too.

I've a question about secondary exes... say, if i install a mod that makes you use a secondary exe that's not recognized by Galaxy (like Bethesda's mod launchers), it's enough for me to just choose that particular exe in the menu? I've been looking around that but I haven't tried it yet, so I'd like to know if someone has done that before.

In Galaxy, there is a setting to add exe's for mods and stuff. That way you can still get achievements and add to play time. I'm not home so I can't check, but I think it's under configure maybe?

If your asking if there is a mod workshop, then no, and that’s a good thing. No platform should have sole control over modding. If you are asking can you mod games from GOG, yes, just as you always have using the free and open moddb and nexus. How long before all those mods disappear behind the steam paywall, or creators club paywalls...

A little offtopic: since the Forum Regulations sticky post is locked, can you edit the first post to include yourself and another one, or notify someone else to do it, to keep everyone in one place?