As most of you know Gmod is known for it's poor optimization especially when it comes to a gamemode that was never really meant to run on it. I've compilled a list below with commands that you can shove into your autoexec file to drastically improve your gameplay. In most instances you'll get double the FPS, unless you have a native dual core such as the Core2Duo series.
steamapps /common/garrysmod /garrysmod/cfg
Find the autoexec.cfg file and open it with Notepad++ preferably which you can get for free here:https://notepad-plus-plus.org/