I had the same problem, and did as black1333 said.
I reinstalled Gmod, and checked that the error didn’t come up there, which it didn’t, so It had to be an addon. So I installed all my addons and deleted them one by one, and after each deleted addon I started the game up and checked.
And I found out that (the goddamn last addon :P) was the one making the error. For me it was:
Mckays Stargate Addon V4.8
If you have that installed, you might wanna try and delete it and check if the errors still there.
If you still get the error after that, or if you haven’t got that addon, the only thing I can say is. Start deleting one addon at a time, and remember to go into the game and check if you got lucky and deleted the “bad” one.