Error message when trying to load Gmod

The message says:

SteamStartup() failed: SteamStartup(0x0012E584) failed with error 1:
failed to take master pipe connection lock

So does anybody have an idea of what could cause this?

You probably need to update your drivers. Look on the wiki on garrysmod.com, there’s a link there where you can quickly do just that.

You don’t. Restart steam.