If I buy a game on GOG, can I assume that I can't access any mods for that game which may be on Steam? If so, that's too bad because I would like to give GOG my business.

You might want to look at Nexus Mods: https://www.nexusmods.com/

Also Mod DataBase https://www.moddb.com/

Also: https://www.gog.com/forum/shadowrun_series/how_to_get_steam_workshop_mods_without_using_steam/page1

You can also try this if you want to download some mods. Some games might work.

http://steamworkshopdownloader.com/

I use Nexus for almost all my modding needs, and haven't had any issues with modding my GOG games.

I suggest this, more organized list of games with Steam Workshop mods. Not that it's been updated in a while, so it's buried, which means it won't ever get updates, etc. Maybe it should be merged with the 2nd class citizen list (at least for those games that have been verified to not allow workshop downloads), since that keeps popping back up for air.

That's assuming you're talking about workshop mods. You should probably be more specific.

Depends, there are 4 type of mods:
1.) Only available on a mod sites like nexusmod
2.) Available on Steam and on mod sites too
3.) Available only on Steam, but you can download it with workshop downloader
4.) Available only on Steam but you can't download it with workshop downloader

1-2.) You are fine. I would say this is the majority of the mods.
3.) You are probably fine, but there is a little chance for compatibility problems.
4.) You can still ask the modder to give you the files or make them available somewhere else.
Or ask somebody who owns the game on Steam to download and give you the files (I'm not a scientist, so I dunno it is legally acceptable or not).

Reminds me of Dungeons of Dredmore, had a bunch of mods that are inaccessible from the HB version of the game. So i had to get steam, use it, download the game, get the extensions, then save those separately so i can run them without relying on steam again.

And naturally putting back in instantaneously created lockpicks...

Reminds me. DoD doesn't like optimized PNG files, making it difficult to make smaller and more optimized... Grrr...

Thanks for those suggestions. I have become accustomed to using the workshop mods on Steam with the 1 click approach. I was hoping that I could still use Steam workshop mods on a game that I bought on GOG.

Nexus is always my first stop for mods, but there is always a chance (depending strongly on the game) that you might be able to salvage the required files from your steam install directory and copy the files manually if you know what you're looking for.

NexusMods is a Disease!

Okay, I'll bite. What's got your panties in a twist about Nexus Mods?

I think he somehow managed to get banned there lol

Can't say it would surprise me.

Because there's a lot of cheats and nude mods and companions that don't make any sense in the world/story?

That's about the only thing i can think of. Lots of nudes... lots of titties... And probably some dongs too.