Yes because you can't play Garry's mod without steam.
Yes, you can play online multiplayer.
no because garrys mod requires that you have a game that uses the source engine
i recommend the vavle pack valve is the creators of steam if u have good gameing PC tho and garrys mod but u must have some of the valve games to play garrys mod great game!
Yes, In "Offline Mode".
yes if you have a legal copy from steam or from a shop you can i have purchased CS:S and Garrys mod together off steam so i can play Gmod.
Use steam download it from their official website.
You can't.
yes you have to have steam.
Garrys Mod 10 Is available on Steam, Which can be found at SteamPowered.com You First need to get a Source Game, which can also be found on Steam, that is including Counter Strike: Source. All and all it should cost about 21 US Dollars.
yes but it is illegal and you can't download addons steam can also trace you if you play gmod without steam for prolonged periods of time so I recommend not using a pirated copy of gmod. it is also likely that you can't play online based on other reports of pirated copies and besides nobody likes a pirater
No, only if you want to play Duels of the Planeswalkers.