I think the Gorosei are kings of 5 of the original 20 Kingdoms of World Government. As we all know, the World Government is somewhat based on the United Nations organization. UN was formed right after the WWII, while the WG was formed right after the Void Century. Now, under UN, there is this what we call the United Nations Security Council and it has 5 permanent members. The following text is from the United Nations page about the Security Council: Now, it can be assumed that in One Piece world, there were 5 kingdoms during the Void Century that led the victory of the 20 Kingdoms against the Ancient Kingdom. And when they formed the World Govenment, those 5 kingdoms' new royalties formed themselves as the Gorosei. Of course, since those nations are powerful, everyone agreed. PS: There's something that suddenly bothered me that has something to do with the Void Century and Will of the D when writing this one. I need to refresh my history class first and do some research and I might post another theory about it. It has something to do with the 2 world wars.