I lag too at times, just the size of the map and all of the custom content.
If you don't know about it already, there is a way to improve performance on Garry's Mod.. Sometimes double the FPS.
Steam Library, right-click Garry's Mod, Properties, Set Launch Options, put '-dxlevel 80' without the quotations. It basically sets the Direct X level to 8 rather than 9 so it doesn't use the more CPU/GPU dependent features and other stuff that DX9 uses. No graphical difference though but some of the post-processing effects won't work with the lower version, as well as HDR.
Lower resolutions will help aswell, but I'd recommend not going below 800x600 as it messes up the GUI depending on the gamemode and/or server.