AND TURN ON ANNOTATIONS ON THE VIDEO!
If you like watching let's plays and gameplay/commentaries, I suggest you to check out my new channel where I post that.
This video was created BEFORE you had to login through Steam on http://garrysmod.org
If you do not own the game on Steam, garrysmod.org will NOT work for you. Therefore I have links further down in the description you can use to download addons. The steps for the installation still works for both non-Steam and Steam versions of Garry's Mod.
This will most likely not work for Garry's Mod 13 non-Steam (if that exists), only GMod 12 and before. This is due to Garry implementing workshop items into Garry's Mod 13.
TL;DR: http://garrysmod.org doesn't work for non-Steam, use links at the bottom of the description. This is only if you have a non-Steam version of Garry's Mod that is before GMod 13.
Even though I uploaded this video for Non-Steam users, I suggest you buying Garry's Mod on Steam here: http://store.steampowered.com/app/4000/
If you do not own any Source-game from before (Half-Life 2, L4D, CSS etc.), I suggest you buying the package with Garry's Mod & Counter-Strike: Source, here: http://store.steampowered.com/sub/219/
Music name: Phoenix Wright - Pursuit ~ Cornered 2001 Cadenza Remix
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How To: Download And Install Garry's Mod Addons [Non-Steam]
For the non-Steam version, you have to go to your Garry's Mod directory. This is not the same depending on how you got it, but it could be C:\Program Files\GarrysMod\ (Program Files x86 for 64-bit Windows users)
If you've found your non-Steam directory, go to garrysmod\garrysmod\addons.
On the Steam version it's a little bit different go to C:\Program Files\Steam\steamapps\*YOURSTEAMNAME*\ga
Recommended site (Thanks to MrDevGuy): http://gaddons.tk
Other addon sites: