1. Click this link and download the "fpsboost.cfg".
www.mediafire.com/download/bejq9hf2dtvbld8/fpsboost.cfg
2. Open your "common/garrysmod/garrysmod/cfg" folder and place the "fpsboost.cfg" in the folder.
3. Start garrys mod and at the start screen (Note: Not while in a server or you will crash) open your console and type the command "exec fpsboost" then hit enter.
4. It will load your screen may go black for a second or two, once done close your console join a server and enjoy the fps boost.
Try this it should work!