The game uses chromium instead of awesomium to render HTML objects, Chromium is pretty much better in every way, speed, stability, and security. It also allows the game to run i 64 bit which increases performance a lot, so its highly recommended.
In order to do this you must have your game closed, then you will need to open steam, right click garrys mod and go to properties.
Now click on the "BETAS" tab and click the slider, now you should use the one called "x86-64 - Chromium +64-bit binaries"
After you have clicked it the game will automatically download chromium and when its done you can go ahead and run the game.