Game screen resizes to corner on TF2 and Portal Maps

Hello, I have a problem with playing Garry’s Mod on Team Fortress 2 and Portal maps.

After it loads up everything, the gameplay is scrunched up to a very low resolution in the top left corner, as well as producing a hall of mirrors-like effect for everything below and aside it.

This is what it looks like:

It happens on any resolution, while on Direct X9.
This however, does not happen when I set the DirectX level to 8.1, but it does make screenshots/videos a lot less appealing to the eyes.

Is this something I can fix? Or a problem with my graphics card?

EDIT: Changing resolutions mid-game doesn’t remedy this, and sometimes causes an hl2.exe crash.