The latest one, Garry, the mod's creator keeps garrysmod updated. The current latest version in the Source SDK is 2009, though recently the "Source Engine MP option has become available" so use that if that is what the question is about.
no because garrys mod requires that you have a game that uses the source engine
No you need the full game or any other source engine
Yes
In order for Garry's Mod to work you need a minimum of one Source Engine game eg. Half-Life 2, Team Fortress 2 or Portal.
Well, in the steams list of games which you can use on garrys mod it dosent show Half life Source. But if you type in gman squad on youtube you can see their playing the storyline on Half life Source. I asked them the same question and they said yes, you can play the storyline if you have garrys mod and Half Life Source. I dont know for sure because i have not yet tried it.
No
Buy Garry's mod 10 off of steam.
Check your "Options" to make sure the spray you want is still set.
It depends on what the player does i guess But its mostly what Half Life 2 is rated If the player has counterstrike source also installed the content (Characters and guns) gets transferred to garrys mod so they can play around with it in the game, it depends on a few factors (1) what will they do? (2) what other games that rely on the source engine do they have installed? (3) what do you think is suitable for them in garrys mod, they are only limited by their imagination, in Garrys Mod you can build pretty much anything, and i mean anything
Garry's Mod 10
technically yes, there were a few updates it is still called Garrys mod 10 though
Through source engines. If that is not what you meant then check menu and look for guns or something.