Yes. I does require System 10.5.8 and Snow Leopard 10.6.3. It might not run on Lion or Mountain Lion. Also, the graphics card must be NVIDIA GeForce 8 or higher, ATI X1600 or higher, or Intel HD 3000 or higher, a mouse, keyboard, and internet Connection.
Yes, the MacBook Air can play this game.
Yes
Garry's Mod is only for Valve-released games, so no.
Buy Garry's mod 10 off of steam.
Yes, garry's mod will work with portal and you will have all the HL2 stuff.
technically yes, there were a few updates it is still called Garrys mod 10 though
Garry's Mod 10
It may work but it will be extremely slow and likely to crash a lot.
no because garrys mod requires that you have a game that uses the source engine
Garry's Mod has now updated and includes all features of 13 Beta
Garry's mod tornado
Steam
yes
You Do Not Get Garrys Mod Serials, You Buy it Off Of Steam Which can be found at steampowered.com its usually about 21 United States Dollars