1. When Ulti about to use "Ulti-meteor", luffy said "Not Happening, Gear Fourth". That might be an indication if he know how big the damage he take if he doesnt use Gear Fourth.
2. When Luffy said "Huh?", his eye literally still on Ulti. (He might know what happen to Ulti later).
But, Ulti seeing the other direction, exactly the direction where Yamato gonna used Raime Hakke to her.
From my point, that two might be an indication if luffy used FS here. But if we comparing his FS to Katakuri, ofc he still loses.