Do you tolerate dairy? If you don't avoid all blends and whey and buy an isolate or vegan.
This will eliminate any probability of a dairy/lactose issue.
In that case
Iso100 Dymatize
Rule 1 Isolate
Nutrex Isolate
Gold standard isolate
Kaged Isolate
New Xtend Pro coming out soon will be a good choice
PES Vegan
SAN Vegan
Both good choices for a vegan protein
Also GNC Brand is not really a great choice when it comes to protein there are far better 3rd party tested companies that are the same price or cheaper and give you a much better product. You could try some blends that have added digestive enzymes. Most GNC Stuff is overpriced and underdosed, thats about 95% of their lineup. Do not shop there and buy from those salesmen that push their house brand when you can get better products.
For example:
Sparta Whey
PES Select
Animal Whey
These settle pretty well for some people even with slight lactose issues.