So, I don't really have any money right now. I'm probably going to trade in a bunch of games to get Portal 2, but I want to know if I can get the game for PC through gamestop and then use it on my Steam account. I'm especially curious because I'm pretty sure the coop will go straight through Steam, so if I can't get it at Gamestop and then just put in a code or activate it for steam, how does the coop work?

If that doesn't work I'll probably just miss out on the 30% extra trade in value and try to get a few extra dollars somehow.

Portal 2 will only work if you have Steam installed so to put it bluntly:

It's a Valve game. Steam is a sure thing.

No mattewr where you buy it, it will have steam.

However, I recommend trading in for the PS3 version because it includes the PC version for free. Then you can just sell/eat the PS3 version.

Ah yes, Gamestop. Kiddies first pawn shop.

Most games now require Steam activation/registration even if they're not bought on Steam...Portal 2 is a Valve game. All Valve games will require Steam, so yeah you're good.

If you're hurting for money, buy the PS3 version, install the PC version, then hock the PS3 version.