This is a short guide I know, but it's worth it.
1.) First copy this line of text and paste in in a .txt document:
(Don't copy here)
cl_threaded_bone_setup 1
cl_threaded_client_leaf_system 1
r_threaded_client_shadow_manager 1
r_threaded_particles 1
r_threaded_renderables 1
r_queued_ropes 1
studio_queue_mode 1
gmod_mcore_test 1
// mat_queue_mode 2 // Dies
(Don't copy here)
2.) Then name that .txt: boost_on
3.) Then find the garrysmod directory, then place it in: GarrysMod/garrysmod/cfg
3.) You're done! All you have to do when you are in Garry's Mod type:
exec boost_on.cfg
or
Paste the code into the autoexec.cfg file in the GarrysMod/garrysmod/cfg (This is so you don't have to punch in the code everytime you start Garry's Mod.)
And watch as your FPS increases! (most of the time.)
*Forgets to add what it actually does.*
What it does:
This basically increases the amount of cores the game runs on, like roast said (thank you for putting the answer down in the comments and reminding me.) The game default runs on one core, but with using this, it runs on multiple. Me personally I have had no problems at all using this method, but everybody is different. And it's not fully compatible and will most likely never be. So I advise you only to use this at your own hand.