Hello I am trying to play SS4 mod Shattered Isles I downloaded the mod from Steam Workshop and put the .gro file in the correct game folder. But it is not showing in the game. Has anyone played the mod on GOG version? Or does someone has a solution how to play it on GOG version. All help is appreciated. Thanks in advance.

Looks like there is still no chance.

Unfortunately, Serious Sam 4 still doesn't have the unrestricted.exe in the GOG release. Meaning it's impossible to play with mods, even if they would work exactly in the way you describe above (just putting the gro file in the proper place). And I doubt Croteam is gonna do something about it, it's been too long.

Funnily enough, Siberian Mayhem does have its own unrestricted.exe, even in the GOG release. So any mods for that should be fine.

Sorry for the necro, but since the 1.09 update of SS4, the GOG version now contains the unrestricted executable, which basically launches the moddable version of the game.

You can follow my guide to download mods from the Steam Workshop and install them on your GOG copy of SS4: https://www.gog.com/forum/serious_sam_series/serious_sam_4_downloading_workshop_content_for_the_gog_version