DISCLAIMER: This only works with the older version of Source that Garry's Mod runs on. Mounting games like Portal 2 or Alien Swarm won't do anything but add unplayable maps. Portal 2 models may work though.

In this guide, I will show you how to fix your problems with your Source games not mounting. First we'll start by going to your games library.

(note: this guide is from around 2013, so it may be outdated.)

Once you're at the games library, it's time to start. We'll use Half-Life 2 for an example. I will right click on Half-Life 2, and click Properties at the bottom of the list that shows up. Once you have opened the Properties menu, there should be a tab labeled "BETAS". I will click on the BETAS tab and then open the list of betas to choose, which there is only one; SteamPipe, the format Garry's Mod runs on. Select SteamPipe and save the properties. Then close the window.

Now, once you have switched to SteamPipe, you launch the game. It won't actually launch it, it will say this:

Steam needs to convert Half Life 2 into a newer, more effiecient format.
This will require an Internet connection and a few minutes' time.

Click OK or START or whatever it is, then watch your entertainment. It can be TV, a book, but not YouTube, because you can't click anywhere once you press ok.

Once you have converted your game to SteamPipe, launch your game and play a map (just to be sure) and then launch Garry's Mod and go to the games mounting list. Your game should be able to be mounted on the game. Now if you go into your maps, if its a Gmod compatible map pack, then you should see all the maps of the game. Well, except for Portal 2 which for some reason has a really disorganized map pack. Hope you're helped with this guide. Bi nao.