Luffy's current peak power is Gear ... That said, this power has nothing to do with Gear 5 and it will see him grow in a very different way. So, while Gear 6 will certainly not exist in One ...

By Punk Hazard, he was hardening his Gear Second and Third moves ... Conqueror’s Haki is different—it’s something Luffy was born with, not trained into. He first used it without knowing ...