You can sue anyone you want for anything you want.
If you have any hope of winning, you need to provide solid evidence that you were directly negatively impacted by the company because of your choice what to do with your own money.
As long as the items you were buying were not illegal, they have no business in what you spend your money on.
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.
Sue them? Over that?? Sounds ridiculous but then again so many lawsuits are so if you want to waste your money on a lawyer and sue them go right ahead.