However, make sure that wherever piece of content it is that you're installing (Episode Two, in this case) is not pointed towards the server directory that you run your server from. Doing so will forcibly install game content to the root folder itself and can cause mass overwrites and issues usually resulting in having you start over from scratch.
Technically - you do not need to purchase anymore games. However, you will need to purchase and download (possibly) Counter Strike: Source and Team Fortress 2 for a lot of the textures and other game objects.
Counter Strike: Source is currently $19.99 at posting. Team Fortress 2 has been free since June 23, 2011. It's important to note that some others games that may be needed are basically any game that run on the Source Engine developed by Valve. This includes games like:
...and many more! But the majority of the textures are handled with just Counter Strike: Source. I would say at the minimum, you will need that game (and you might as well get Team Fortress 2 since it's free). A complete list of Source games can be found here.
By not having the correct games, you could potentially be missing the textures needed to play maps created by others. You will see a lot of checkered purple and black objects, and actual "ERROR" messages like this:
In addition to the previous answers, it is worth noting that some maps or mods have models/textures compiled with them. This means that when you download (for example) a map, even though it may have (for example) HL2 Episode 2 textures/models, you are still able to see them properly without the game installed because you have downloaded them with the map. Some addon/level creators have even used custom textures and models that you will not find in any other game.
The other games are not required to download a map/addon, and upon joining a server with a map with these things that you do not have installed, you will still continue to download the map, however errors will still arise (and you may get script errors, but these usually aren't anything to worry about).
Some addons for Garry's Mod require that additional content from other Valve source games be made available, in order for those addons to work correctly. This content is used by certain game modes (DarkRP being the most common one) to provide missing information for props and physics information needed by the game for it to work correctly.
Select all the files, and use a utility program to "zip" them up, so that they can be copied and moved easier. If you don't have a utility that does this, we suggest using 7zip, as it's free, and very easy to use. You can download 7zip from here.
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The First Workshop Content Pack was the in-game release of several items that were selected by Valve for outstanding quality through the Steam Workshop. Released with the September 6, 2012 Patch, the update added a total of 9 new Cosmetic items. They were only available via opening Fall Crates with their special Fall Keys. These items were added to the item drop system at May 17, 2013 Patch. 2b1af7f3a8