Text guide:

NOTE: If you are on a 64 bit OS then you will have to replace the lines C:\Program Files with C:\Program Files (x86), if somethings not mounting, it's probably because you didn't point the cfg to the correct program files directory.

Go to C:\Program Files\Steam\SteamApps\common\GarrysMod\garrysmod\cfg and you'll see a file by the name of mount.cfg

(Make a copy of this file incase you make a mistake)

Open this file, and it will look something like this:

//
// Use this file to mount additional paths to the filesystem
// DO NOT add a slash to the end of the filename
//

"mountcfg"
{
// "cstrike" "C:\steamcmd\steamapps\common\Counter-Strike Source Dedicated Server\cstrike"
// "tf" "C:\mytf2server\tf"
}

Now type this line above the bottom bracket,

"MOD1" "C:\Program Files\steam\steamapps\sourcemods\modname"

replacing the last part (modname) with the appropriate mod folder name, the first part (MOD1) can be whatever you want it to be for reference. It should now look something like this:

//
// Use this file to mount additional paths to the filesystem
// DO NOT add a slash to the end of the filename
//

"mountcfg"
{
// "cstrike" "C:\steamcmd\steamapps\common\Counter-Strike Source Dedicated Server\cstrike"
// "tf" "C:\mytf2server\tf"
"MOD1" "C:\Program Files\steam\steamapps\sourcemods\modname"
}

Save the file, and you're done, unless you want to mount more mods, just duplicate

"MOD1" "C:\Program Files\steam\steamapps\sourcemods\modname"

above the bottom bracket, and replace modname with the exact name of the mod's folder name, and MOD1 being renamed to whatever you want, for reference.

OPTIONAL:

However, if you want to mount Half-Life 2 and it's episode's content, minus the maps (have Source SDK 2013 Base Singleplayer installed), your mount.cfg file should now look something like this:

//
// Use this file to mount additional paths to the filesystem
// DO NOT add a slash to the end of the filename
//

"mountcfg"
{
// "cstrike" "C:\steamcmd\steamapps\common\Counter-Strike Source Dedicated Server\cstrike"
// "tf" "C:\mytf2server\tf"
"MOD1" "C:\Program Files\steam\steamapps\sourcemods\modname"
"HL1" "C:\Program Files\Steam\SteamApps\common\Source SDK Base 2013 Singleplayer\hl2"
"HL2" "C:\Program Files\Steam\SteamApps\common\Source SDK Base 2013 Singleplayer\episodic"
"HL3" "C:\Program Files\Steam\SteamApps\common\Source SDK Base 2013 Singleplayer\ep2"
}

And that's it, you're done!