What the Gorosei's motives are will likely tie in to the truth behind the Void Century (exactly what the World Government did to seize power in that time that they want to cover up), so it's likely not a question that will be answered until Raftel. I would imagine the Gorosei are the Ministers to the Tenryuubito's Monarchs, as it were: the Tenryuubito are nominally the rulers of the world, but the Gorosei wield the functional power while the Tenryuubito just dick around, buying up mermaid-slaves.
Given that the One Piece world in general and the World Government in particular seems to operate on the precept of "physical strength = rank" (in every case except Spandam that has been shown to be the case), so the Gorosei would at least have to be on Garp's level or so, though it would be fun if they were just that much stronger than the Admirals
Monster Hunter: pissing me off since 2010.
