Ya know, I could sworn the GS were revamped G, guess I was wrong. Thanks for the correction ko888.
You don't need insider info to understand that 3rd party brands call up OEM builders, spec out a psu platform and make a couple changes. Sometimes they'll even send a design to the OEM, who then builds it for them. Sometimes this has great results, like the Evga G2's built by SuperFlower, sometimes not, like the Corsair RM 750/850 built by Chicony.
Either way, OEM just builds what the buyer wants, whether it's a rebranded copy or brand new design.