Normally Garry’s mod runs as a 32 bit application for this reason the performance suffers. With a simple trick, Garry’s Mod can be started as a 64 bit application and more than double the performance, especially on large maps.
The following steps are necessary:
1. open your Steam client
2. navigate to your library and search for Garry’s Mod
3. right-click on Garry’s mod and open the properties
4. go to the menu item Betas and select the version “x86-64 – Chromium + 64-bit binaries”
5. close the window and wait until Garrys Mod performs the update
6. when the update is finished you can start Garrys Mod and select at startup that it should be started as 64 bit application
7. Now you should have a lot more power when playing Garry’s Mod have fun