First we go to our library, and then we locate the game.
Click with the right mouse button and a contextual menu will appear, in which we must select the "Properties" option.


After this, a window should appear (containing the respective properties).
All we have to do, is just select the "Betas" tab, and perform the steps like the next image:


Once all the steps have been followed, a small update for Garry's Mod will begin to be downloaded.
After it's updated, we can play with mods and others things that require the power of "Chromium".