• Start up Garry's Mod (or another source game)
  
• Open the console
  
• Type this: mat_dxlevel 90
  
  • Note that you can go higher if you want but your PC can only handle so much and some DXLevels only work depending on what version of Direct X you have installed
• Start a map
  
• Try your stuff out

Here's a video instead:

Go to your steam library and right click on your gmod then click on properties then click on Set launch options, after that type into the box -dxlevel 90. If that does not work, update Direct X. Your computer may not be detecting your DX 9.

Or, you can simply buy a new PC because if you're having to do everything in this guide then your PC sucks really bad.

NOTE THAT ADDING THIS TO YOUR LAUNCH OPTIONS WILL CHANGE YOUR RESOLUTION! I CAN NOT FIX THIS AS IT IS A PROBLEM WITH THE STEAM LAUNCHER ITSELF!