Edit- In case anybody else is thinking of getting Gears of War for PC second hand:
 
You can play the single player campaign offline without a product key, as long as you make an offline profile.
 
If you do already have a Windows Live profile, but the games CD key has already been used or is missing, you CAN use another 'Games for Windows Live' product key.
 
Hope this helps.

For a lot of PC games, you can use the CD key on multiple computers. Sometimes you have to de-activate the game on the first computer and then activate it on the second, but i'm not sure about this specific case. 

It's a weird GFWL thing. You need the code to attach the game to your account, but you can still play single player in offline mode...or something.

Also, there is a weird patch bug where it can be very difficult to get the game running depending on a few things. I know that I can no longer run the game, but I don't remember exactly why.