Unsure, try grabbing a better graphics card or free up some space. It is also possible that your garrys mod is loaded with addons and such that might be causing lag. I am unsure of the problem as I get around 60-70 FPS when I play PERP. Wait for a coder or something to try to figure this out.\\\
PSSST: ALSO CHECK YOUR RAM! MAKE SURE TO HAVE AROUND 6 GB RAM INSTALLED AND BEING USED! ALSO, YOUR OPERATING SYSTEM MIGHT BE IT. IF YOU USE 32 BIT THAN THAT IS MOST LIKELY THE PROBLEM! 64 RUNS BETTER AND MAXIMIZES PERFORMANCE!
Hope that helped