I get this error every time I try to launch the game. I can barely get past the “Preparing to launch Garry’s Mod” screen before it pops up.

Anyone know how to fix this?

Restart Steam, if that don’t work then your computer.

check specs, run dxdiag.
post them, see if you can run it

You could also try deleting ClientRegistry.blob in the /Steam/ folder then restarting Steam.