Certain mods contain assets from other games, however that does not mean the game part is a literal part of the mod. Some maps use assets from other games, but has all of the textures packed in. To get started, we are going to use Team Fortress 2 as an example, we will use the Team Fortress 2 Bots mod First, install the mod and check if all models are errors, this is when you know: You don't have TF2 or it's not mounted.

First, be sure you have the game you need downloaded (tf2, portal, portal2, l4d, day of defeat source, counter strike: source, ect) Now, go into gmod and click the controller button at the bottom right of your screen. You should see a list of source engine games including the one you need. Click the box next to the game, you should see a check mark appear in the box, yay... The game is mounted and errors should not appear on mods that uses the game assets.