Hello bargoo- I'm glad you like GNC. There are plenty of pros and cons about working there, and a lot of pros and cons about being a consumer there; but I'll start with the basic beginning to finish with finding a protein supplement. When you enter the average GNC store (unless you're lucky enough to be near a franchise) you'll be greeted and such. Then they'll ask you what you're looking for, usually by saying "What can I help you with/find?" or the more corporate acceptable pitch, "What brings you into GNC today?"
As soon as you ask for protein, you'll be led to the Wheybolic Extreme 60- this is a P3 product. We don't get comission on this product; however, it does benefit store percentages, so it will be shoved down your throat. The GNC motto (that only the employees know) is "Don't quit selling until your customer lets you" - so be careful. Employees aren't evil, and I don't want to paint that picture; but they do have plenty of bosses above them that pressure them. We keep our job based on percentages, so it's just business.
I would politely refuse the Wheybolic, and take a look around for yourself. Are you worried about calories? Try Designer Whey - it's a great brand with a wonderful reputation that comes in some decent flavors - only a hundred calories per serving. Are you worried about the blech taste of protein? Try Optimum Nutrition or some of the GNC brands - they aren't bad. I have cookies and cream in the 100 Percent Whey (GNC brand), and it's awesome with skim milk and ice.
In the end, the protein you pick will be a personal preference - make a list of expectations you have for your supplement and start from there. Some people come in the store very adament about no artificial preservatives or flavorings, some people prefer liquid protein, some people want the cheapest, some people want the smallest container to start out with.. there are so many things to consider, that you'll just have to take your time and look. Try to weigh every option.
GNC employees are trained. We take tests quite often, and we study products or clean them when no customers are in the store- I know I do, anyway! So asking the employee is not a mistake - just be careful. If you feel like he/she is making a sales pitch or pressuring you, say "Thank you, I'd like to look around now" or something to that effect.. but if you feel like they are only informing you of the products, it's perfectly fine to trust them.
I would look for any sample packs, too. Some stores carry those in the GNC brand proteins.
Good luck, and if you have any more questions, please ask.