I remember "back in the old days", Gregory allowed for the main pack's hip belt to be completely removed, then slipped thru the underside of the top lid, thus creating a fanny pack with a massive hip belt. Competitors typically "opted" for the tiny waist belt tucked underneath of the top lid, making for a much easier system.
I've been so out of the loop on Gregory products for a while, so I have no idea if this is still the case for their packs. Frankly, I seriously doubt it, given how "specialized" frame systems are these days. One "clue" could be if you can run your hand thru a "slip" on the underside of the top lid. If so, I'd suspect they are providing for some form of hip belt to work with it.