Multicore Processing option not working

I load up Gmod and everything works fine but I want it to use both processors when I play. In any other game the Multicore Processing option in advanced video can be set to enabled, but in Gmod, the drop down menu has nothing in it. How can I set this to enabled?

You can’t.
Due to Garry’s Mod using lua, you cannot use more than one core.
Use it to your advantage.
Set Garry’s Mod and steam on one core, and other processes on another.

Well shit.
I finally have a dual core system (sadly the best I’ve had) and I can’t utilize it with my favorite game.
How fun.