How?
Right click Garry's Mod in your Steam games library, click Properties, click on the Local Files tab and click 'Verify Integrity of Game Cache'
Then wait until it says "Completed"
If that hasn't helped, it could also be an addon that's either terrible, broken or may just be colliding with another addon, so you can try disabling all of your addons and see if anything happens, then try re-enabling them one by one and restarting your server after each one
