Yes, any game published by Valve requires that you use their content service, Steam.
In order to download zombie panic from steam you must have Half life 2. So yes, you do need half life 2
You need to buy them, usually from the store or from Steam.
Since Half-Life: Opposing Force is an expansion pack, you first need Half-Life and a Steam account. Pay the $4.99 required to own the game, then download it through your Steam client.
You need half life if your want counter strike and Steam if you want to play online you can also buy it from Steam..
Garry's Mod requires only that you have one Source game.This includes Half-Life 2, Half-Life 2 Episode One, Half-Life 2 Episode Two, Team Fortress 2, Counter-Strike: Source, Half-Life: Source, Portal, and a few others.However, if you have Half-Life 2 Episode One, you will only be able to experience Half-Life 2 Episode One (and Garry's Mod) content. You won't be able to fight alongside Griggs and Sheckley with a home-made "Antlion Dispatcher", or be teased with cake while solving complex puzzles using the portal gun, physgun and toolgun.Long answer short, you should only need Half-life 2 Episode One in order to play Garry's Mod, but you can experience more of Valve's games in Garry's Mod if you have all of Half-Life 2.
Yes, you do. You need steam and a source game such as Counter-Strike: Source. Or Half-life Source. Day of Defeat Source
Yes you need half-life
no, but you do need a source game like Half Life, Half Life2, or Team Fortress2
Yes you do. Counterstrike: Condition Zero, however, is not a mod of Half Life, and therefore, does not need Half Life to acquire.
Any Source game. This includes: Half-Life 2 (You pretty much get stuff from this in any Source game) Half-Life 2: Episode 1 Half-Life 2: Episode 2 Team Fortress 2 Portal Counter-Strike: Source Day of Defeat: Source Get any 2 or more of these and you should be set to go, but I really honestly suggest getting all of them, its only about $50 (if you get the $30 Orange Box, which contains all Half-Life 2 games, Portal, and Team Fortress 2, then get CS:S for $20) Alternatively, Team Fortress 2 is now Free-to-Play and downloading that will allow you to play Gary's Mod, or any other Source mod!
No you cannot because Counter Strike 1.6 runs on the original counter strike engine. Every game on garrysmod must run from the Source Engine or any game ran off of HL2. From my memory, These are the requirments. You must have ONE for gmod to work. Garrys Mod*Required Half Life 2 *You need this for basically everything in Garrys mod Half Life 2 Episode One Half Life 2 Episode Two Counter Strike Source Team Fortress 2 Half Life Source Day of Defeat Source
Well, if you bought the PC version though Steam, you can download Source SDK (free) though Steam, under Library, Tools, then double-click Source SDK to start the download. If you need any help using it: http://developer.valvesoftware.com/wiki/Main_Page