Steam won't read Gmod.

Okay, I downloaded gmod on a laptop because I can move it around and get a better download speed. I put gmod on a USB and transfered it to my desktop. I put one Garrysmod in steamapps/commons and one in steamapps/username. I then start up Steam and it still says it’s downloading? Can I get some help? I also only have one steam library folder.

make sure you put it in the correct folder

I told you, I put one in Steamapps/Commons and Steamapps/Username

Try copying the appmanifest_4000 too (make sure Steam is closed though).