I've noticed the offline installers add a bunch of extra files that seemingly have no use, it should be safe to delete them, right?

I know that some custom GOG .exes have stuff needed for older games to function, but for newer games, none of this is actually needed, right?

I'm talking the goggame files, the Galaxy .dll plugins, the unins000, webcache, etc. Attached is Hollow Knight as an example.

At the very least i always delete them and nothing bad ever happened.

No, some games require galaxy dlls.
https://www.gog.com/forum/general/gog_galaxy_required_in_certain_games
Delete other files at your own risk.

Do we have a list of which? I don't mind leaving them in if required, but most of the games I play here are DRM-free on Steam and Humble anyway, so I doubt they'd do anything shady like that.

Before deleting them permanantly try moving the files to a different location first. That way if it all goes to hell in a handbasket then all you need to do is move the files back instead of having to do a reinstall. That's what I do and if all goes OK I delete them then.

Many games often need Galaxy.dll's (even if you're not using Galaxy). Those files intercept API calls from the game to the Galaxy client (eg, achievement unlock) to "silently fail" without issue if Galaxy isn't running (instead of crashing). If you delete them you may well see an error message (see a.png attachment).

Stuff like unins (uninstallers), webcache, support, gog.ico, and most goggame* (Galaxy metadata) can be deleted without issue though obviously if you delete the uninstaller files you'll need to manually delete the game folder to remove it.

Yeah, I'm sure that's safe, but as adviced, try moving before shotgunning.

So it'll just have to be a game-by-game basis? I removed all traces of Galaxy and Steam plug-ins for Hollow Knight, and got myself a charm in a new playthrough to trigger an achievement, no problems there.

From my experience, the following files can be dropped with no side effect: Some other files seem to be dynamically added by the installation process and are probably useless too:

Trial and error, I guess.
Personally, I would put all the Galaxy-related files on a flash drive or SD card and shoot the damned thing into space.
Offline installers, as the title suggests, should have no link whatsoever to the client or irrelevant files in them.
Delete at your own risk, but make sure to have backups first.

Agreed, it's what would make me buy games on Humble over GoG if they're available DRM-free there. I really dislike all the extra garbage that's included. Just a shame so many put their games on GoG and not Humble as well.

Well, if you like command line scripts, you could do a pattern based search and delete, but keep in mind that has to be specified exactly or bad things happen.

I have experience. It seems games have identifier numbers in the goggame- files, so I could use those to identify the game and delete the files if they're unneeded.

Not like I mind doing it manually either though. Just annoying those files exist in the first place in an "offline" installer.

Yes, for some games the galaxy.dll files can be deleted. Others like Deus Ex:HR won't start at all (as shown in the previous picture). There's no way of telling and no pattern to them. It's best to just do what others have said and move / rename them first (you could always create a subfolder called "junk" and put them in there), then test to see if that works to avoid having to uninstall & reinstall.

So generally if a game doesn't work without them, I'll know on launch? As in I won't just get a crash mid-game?