Gmod won't recognize Dystopia

How do you get it to mount? I have it installed but gmod just shows the little folder icon with a ‘-’ on it

It’s a game that uses the old GCF; you can’t mount it.

Only way to use its content is to extract it into garrysmod.

Yeah, the game isn’t updated to use Valve’s new content system. Grab the program GCFScape and manually extract the content by dragging it into your Garrys Mod folders, or making it into an addon.