Ok, I have the file now but I can't find the mods folder in GoG's Linux version.

Steam/Windows: C:\Program Files (x86)\Steam\SteamApps\common\dont_starve\mods\
Standalone/Windows: C:\Program Files\dontstarve\mods
Standalone/Mac: /Applications/Don't Starve/Don't Starve.app/Contents/mods/
Steam/Linux: /home/username/.local/share/Steam/SteamApps/common/dont_starve/mods

GoG/Linux: ?

I've found
/home/username/.klei
but it doesn't have the games files and putting the mod anywhere in there doesn't work.
--------------------------------
Edit:

Ok, I'd have never thought that it's installed outside of
/home/username/
but it actually is in

/root/opt/GOG Games/Dont Starve/game/dontstarve64/mods/

That's really odd. Having blank spaces instead of _ is quite unclean too.

Would you mind moving your "Edit:" into a reply post and marking it as the solution so this no longer shows up as an unsolved problem on the forum list view?

Unfortunately I can't mark my own post as a solution.

Sure, I don't mind giving you a rep boost although mine is on a constant decline for months thanks to a small hate-mob in the gamergate topic.

Ugh. That reputation feature really needs to be rethought.