No, unless you let them on your account, which is a bad idea and not recommended.
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You can't.
Garry's mod 9 is free, it was the last free version. it uses steam to launch as it resource files are all valve games. you cant star gmod without steam
No, games need to be installed locally onto the hard drive of the computer you wish to play it from.
Yes you can, you import and add non-steam games to your Steam library. From within the library you can launch and run any games, although 'Steam achievements' will only be available for Steam enabled games.
Yes, some. Not all of them can be activated on Steam. See the related links for a list of the games that can be activated on Steam and for the instructions on how to activate a game on Steam.