There is no denying the fact that Luffy ... Regardless of whether Luffy hits them with his strongest Gear 5 attacks or his strongest Haki-based attacks, he simply cannot win.

Gear 6 is unlikely to exist, as Luffy has stated Gear 5 is his pinnacle devil fruit form. Luffy's future growth will focus on improving his Haki abilities ... the Hito Hito no Mi, Model: Nika.