SOLVED, TURNS OUT I’M JUST STUPID! CLOSE THIS PLEASE!
First of all I’m sure I’m running in DX9 mode, so that’s probably not the problem.
But materials that have a shiny sheen (like black_chrome, chrome, and other metallic textures) have lost their shine. black_chrome and chrome respectively are plain old solid black.
I haven’t played Garry’s Mod in a few weeks, so I’m not sure what could have happened.
This is all I can offer, if you need any other info I will supply it.