For those who don’t own Counter-Strike: Source, Half Life 2: Episode 1, or Half Life 2: Episode 2 playing Garry’s Mod might be kind of a pain. Here’s a link to addon packs you can extract into your addons folder to side-load this content.
Extract the folder within the archive into your addons folder. Historically this would end up being something like below:
C:\Program Files (x86)\Steam\SteamApps\common\GarrysMod\garrysmod\addons
Counter Strike: Source
Half-Life 2: Episode 1
Half-Life 2: Episode 2