Is it just me, or is Luffy "Knocked Out" on the ground, yet still in Gear 2nd?
His body still seems to be steaming. So he is hurt and on the ground, but he can't be completely unconscious if he is still in Gear 2nd. Just on the ground in pain.
Heck, I could see Luffy commenting next chapter that he is in so much pain from the usage of gears, rather than Lucci's actual attack.
Either way, if Luffy really is still in Gear 2nd like it appears. He'll be conscious enough to hear Usopp and have another wind to make a comeback.