Huh?
You work at GNC, and they tell you what to buy or not to buy? That does not sound right. Unless, the GNC you worked at was joined with other businesses, in that case they want you to keep your own shopping to thier sore. That's like a person going to orderf ood in a burger king while wearing a mc donalds uniform.
You are free to purchase what you want.
Unless, some items are age restricted and youa re underaged, in that case you should not work their in the first place.