Ok, first off, if you're working at GNC, then you've got racks of protein bars and shelves chuck full of a huge variety of supplements....I don't think you should have any problem finding/having access to something to eat!
Secondly, yes, you want to feed your body every 3 hours or so to provide a steady flow of nutrients. Of course, this is only if you're serious about keeping your muscles properly fed and in an optimal state of growth/repair.
If you find yourself getting hungry within 2-3 hours after you've started your shift, then that's not a good sign...it's your body telling you it's hungry as hell and it's going to look for fuel...if the fuel is not provided, then it will turn to what it can find...which can be
muscle tissue. Believe me, you don't want to risk that...unless you don't mind wasting your effort and time in the gym.
There should be no reason why you can't either pack a few easy-to-consume meals with you or eat some of those protein bars right off the shelf. There should never be any excuses for why you can't eat...it's all about planning ahead and being prepared.