I noticed many people were having issues with mounting games to their server. I did about 2 hours of testing and figured out the issue. Here's a complete guide to how to mount TF2 and CSS to your server.

Step 1. Navigate to the cfg folder (Located in "C:\<go-to-your-server-directory>\garrysmod\").
Step 2. Open (or create if it doesn't exist) mount.cfg.
Step 3. Change the directory of "tf" and "cstrike" to the STEAM directory (point it to your client installation). It should look like this:

"mountcfg"
{
"cstrike" "C:\Program Files (x86)\Steam\steamapps\common\Counter-Strike Source\cstrike"
"tf" "C:\Program Files (x86)\Steam\steamapps\common\Team Fortress 2\tf"
}

Step 1. Create a file named "mountdepots.txt" in the cfg folder.
Step 2. Add the following to the file:

"gamedepotsystem"
{
"hl2" "1"
"cstrike" "1"
"dod" "1"
"tf" "1"
"ep2" "1"
"episodic" "1"
"hl2mp" "1"
"portal" "1"
}

Step 3. Launch the server and watch as the server mounts TF2 and CSS! (You need them installed and launched at least once for it to mount properly.)

-Notes-

• The names in "mountdepots.txt" MUST match the ones in "mount.cfg".
  
• To disable a game from being mounted change the "1" to "0" in "mountdepots.txt"
  
• There must a be directory alongside the name of the the game (e.g. "portal" "C:\Program Files (x86)\Steam\steamapps\common\Portal\portal")
  
  Good Luck and Thanks for Reading!