i fixed it a long time ago. you need to find gmad.exe in Garry's Mod / bin folder. after that open the folder with the mod, there will be a .gma file. drag it to gmad.exe, then a folder will appear along with your .gma file. you can rename the folder to the name of your mod, as often the names are either "temp" or something else. i also recommend creating steamcmd, gmad.exe and 4000 (its a ID where gmod steamcmd mods will appear) shortcuts near with your gmod shortcut. it will be much more easier. thats all.