It says i have pirated Garry's Mod. But i have just bought it

this is solved

Link is broken.

Excuse me: http://steamcommunity.com/profiles/76561198021081220/games

Are you sure theres no other weird steam patching thing that you have on.

It has been a year since i uninstalled it, but i edited the thread and said that it now works, so it could have been an error.

There’s 2 solutions:
1- Restart your steam.
2- Right click on Gmod in steam then go in Properties, Local files then click on Verify Integrity of Game Cache.