Will Gai open the 8th Gate?
The title speaks for itself. Opening the 8th Gate uses up all of the body's energy and forces the heart to pump at its maximum rate, resulting in death for the user.
I think that Gai will open it - he just seems headstrong enough to do it, and Kisame seems really strong, even in a semi-weakened state.
Also, we've seen that in Naruto's new 9-tails form, he releases what seems to be "life energy". It was enough to turn Yamato's wood totems into large trees in a matter of seconds. So maybe if Gai opens the 8th Gate and kills Kisame, Naruto can "heal" Gai back to normal and save him from dying.