I am not sure who makes the private label GNC supplements but the majority of the supplements on the market are made by a few huge companies.
For example Iovate owns muscle tech, hydroxycut, epiq, xendarine, truegrit, and a few other companies
Glanbia owns ON, BSN, isopure and a few other companies
GNC probably has a big company like Iovate or GLanbia make their supplements.