Sand
Gravity is cool and all, but I doubt that it's going to be effective. The fact that Gravity's benefits can only last for 5 turns at most, and the lack of benefits that Gravity actually provides for the effort it takes to set it up makes me feel that making Gravity the center of this CCAT will make it fall flat on its face. Sand, on the other hand, is a relatively unexplored option in RU. We did gain Stoutland from the somewhat recent tier shifts, and Sand Rush Sandslash was released from the Dream World not too long ago, so Sand finally has some reasonable sweepers to potentially make setting it up manually worth it. Sand is also pretty flexible as far as teambuilding goes because it's not overly hindering for most Pokemon in RU, and since there are only two Pokemon (or three if you count Sand Force Gigalith) that benefit from it, there's a lot of room for us to fill in the rest of the gaps with creative and interesting choices.