This is probably if you're running a config that puts Garry's Mod under DirectX9
Try -dxlevel 90 or 95 in your Garry's Mod launch options
You can also look this up in-game and see what comes up if you type mat_dxlevel in the console and see what value it spits out.
Also worth mentioning that you should update drivers and make sure that your actual dedicated GPU is being used and not your integrated graphics.