I have often seen people have this problem when they attempt to install it to the old garrysmod folder from before the steam pipe update, which used to be in "C:\Program Files (x86)\steam\steamapps\accountnamehere\garrysmod\garrysmod" by default on Windows. Ensure that you are accessing your garrysmod folder through the "BROWSE LOCAL FILES..." button of the game properties and that you are installing to the garrysmod subfolder of that. If you are in the correct folder, you will see a file called "gameinfo.txt".

same here
but i think i fixed it
idk though

I didn't fix it.