low FPS on tf2 maps

Even though i can play TF2 all maxed out, whenever i try to use its maps on Garrys Mod my FPS goes to hell (~6 fps).

I’ve already tried setting video settings to the lowest possible, playing on MP, playing on SP, tried googling it and searching on these forums (found nothing), weirdly enough, a bunch of my friends also have this issue.

Textures also get messed up, mixing half-life 2 and TF2 textures together in the same wall (not over eachother, but side-by-side, in a wall where there was supposed to be only one texture), same thing happens to props

This only happens on TF2 maps

My PC:
Core 2 Duo E7400 @ 3.2Ghz
Radeon HD4770
2x2GB different-brand RAM

I remember i recorded a video with fraps about one year ago on 2fort (through gmod of course) and it went smoothly, any idea on whats going on?

Re-install GMod.