Please Help me Garry's Mod Problem

Hello I have this problem on when I go to my games on steam and then click play on Garry’s Mod and then nothing happens and then it say’s I’m playing it but then when I try clicking play again it does nothing it just attempts to start and then it says its already running but nothing is open with Garry’s mod please help me!!!

Check your processes, is hl2.exe running? Try ending the process and opening Gmod again.

If not, I’d recommend verifying your game cache. If THAT doesn’t work, re-install Gmod.

Well I’m running GMod from steam and I’ve tried running it from my desktop and it still happens and how could I re-install GMod I dont think I have the right Steam because everytime I run steam it updates and then downloads almost my whole desktop of files and its so annoying