I really don't know how luffys gonna win this unless he finds a mirror somewhere, gear 4 to buy some time and jump through the mirror. Or he does something like Gear 2nd Max and hits katakuri so fast he can't turn to gum fast enough.
He's definitely going to have to go Gear 4 to win this. Katakuri is able to match everything else he can do, Gear 4 is unique in that his Mochi powers can't mimic it at all given how it works.