Yes, and it's fun and free.
only if you have Half-life: source as the model formats are slightly different, as are the enginesyou can get Half-life source off steamat: < http://store.steampowered.com/app/280/ >
To get Half-Life 2: Episode Two content in Garry's Mod (GMod), you need to have both games installed on Steam. Once both games are installed, GMod will automatically recognize the Episode Two content, allowing you to access its models, maps, and textures. You may need to enable the content in GMod's settings or ensure that the game is checked in the "Game Content" section of the GMod launch options. After that, you should be able to use the Episode Two assets in your GMod sessions.
I think you can use portal in gmod 10, but I'm not so sure.
runOnTick(1)
Yes, you must.
If you want to make models for gmod look at XSI Mod Tool.
download them from garrysmod.org
gmod is for PC only
yes
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
you can use gmod in any way you like its pretty much a free roam game where you can spawn things and download addons the gmod website its an awsome game hope you enjoy! p.s. if you dont have a steam account you cant play gmod and you also need a game source by lolkid007
yes, you can use props/characters from all games that are made by valve