• Half-Life 2: Deathmatch, Episode One, Lost Coast
  
• Half-Life: Deathmatch - Source
  
• Left 4 Dead 1
  
• Team Fortress 2

Since I know that Garry's Mod utilises all of the items and SDK from Source games, I realise that without a sufficient amount of said games it would be rather boring. Will those mentioned above give me a decent experience? Do I need to install them all or just specific ones? I am asking because the game is on sale on Steam at the moment.

Yes, that would make for a good experience. You'll get lots of prop and character variety by drawing from three different game series. Left 4 Dead's characters are particularly fun to pose in my opinion.

Have fun.

Thanks. Do I need all of them installed or just a few?

Team Fortress 2 is free, so unless you don't have the connection for it, you should download it.

Skip Lost Coast unless you can get it in a bundle. It introduces one new level and one new character, and that is it.

Episode One can be skipped, though it contains some levels you might like.

Half-Life: Deathmatch - Source can be skipped. There's really nothing interesting there, unless you're really into the original Half-Life.

Half-Life 2: Deathmatch is pretty much a necessity if you're not getting the rest of Half-Life 2.

And Left 4 Dead 1 contains several great characters and some good props.

So there you go.

I am already almost 50% way through downloading Half-Life: Deathmatch - Source, so I might as well finish it. I'd prefer not to get the rest of the Half-Life franchise, since I own The Orange Box on my Xbox 360, and only own these through getting them free last Christmas from Steam. I'll install the two Deathmatch games, as they appeared fun when I tried them out a little while ago. I really don't like Team Fortress 2 and it seems to lag on my internet connection or something, but I may download it if I can be bothered to waste the time. Since I have been meaning to get back into the original Left 4 Dead, I'll also download that later. Do I need these installed before Garry's Mod, or will it pick them up if I install them later? Finally, is it true that you can download mods which add more items to the game? I think I saw it in a YouTube video...

If you install new games or mods after you've installed Garry's Mod it will detect them all just fine.

I thought Lost Coast was free?

And what about CS:S. I think that's a goodie.

No, it's actually $40 (!) on Steam, by itself.

But if you get it in a bundle it's usually far less than that. One of my brothers managed to get it free with a new copy of Half-Life 2 several years ago, but today I think it costs something like $5 if you buy it with Half-Life 2.

You can't purchase Lost Coast by itself anymore, it says 40 dollars because the only purchasing options are bundles.

Oh, I see. Steam is confusing.

I was about to say. What the hell? It seemed so out of the ordinary for any company to do that let alone Valve.