Gai is a hot-blooded bastard. Don't question him u.u
OT: Him and Lee have always been able to use Gates while in a tired form. The two of them don't let the state of their body effect their fighting much, plus opening the Gates leads to a surge of energy/chakra.
Well, he did use the 7th gate against obito and in just a matter of few hours he was able to use it again. I mean his chakra should be almost nil right?
You remember back when Gaara and Rock Lee fought and Lee got up without actually being at his standard mind set at the time?Well they have a mighty will power. Will of Fire.