most gamestops will have a midnight launch. some are even doing a crazy launch party. i know ithica and a few others are hitting up one that is getting a dj, some drink sponsors and some other stuff.
it will really depend on how many reservations your gamestop has. if they have a smaller amount then they will just hold it until the next day. but seeing as how its a halo game, then there should be a ton of reservations.