Thet don't change roster. It's always the same R kamihime's in gem gacha plus eventually new ones are added to this, like recently 6 new ones (don't confuse it with new ones added each event to the jewel gacha) were added to the previous ones that were there from the start. I believe right now there is about 12 R kami's in gem gacha and I woulnd't expect another ones added to it before 2018.