Its not gonna happen. In reality, its not really needed either. You can predict at this point what the ability will be.... morning peacock, afternoon tiger..... must be evening dragon.
If Kishi did show it, he'd most likely let Gai live, like how Chouji was supposed to die after taking the red pill back in part one and didn't. So when it comes down to it, the 8th gate is not really needed in the story anymore nor would it or its ability be any big surprise.