If you didn't pre-order a game from gamestop, and you go in and try to buy it there, will they have any extras available?
Depends on the Game Stop I guess... the one by my house is almost never busy so I rarely have to preorder.
If someone failed to pick up their copy after x amount of time, then yeah, they might have some. But it's not like they'll just hand it over.
It would probably be best to call the store and ask. Like I said, I think it depends on the store and how many preorders that particular store got. When Black ops came out the guy at mine said they weren't going to have any copies if I didn't preorder, but when I went to pick the game up people were still getting it without a preorder.
I've heard in the UK Game have had to cancel some folk's Signature Edition due to low advisability. Thankfully mine is safe. =D