Well that would all depend on 2 key factors, one being time, and the other distance. Mind you, even though we never seen 5 tails you can imagine that Naruto would be faster and maybe as powerful as he is in six tails form. Now as far as distance, once again be mindful that 4 tails and even 6 tails had some pretty niffty long range attacks. So Gai would have to be very safe distance away from 5 tails in order to open all 8 gates. Could Gai stand up to Naruto in 5 tail for with all 8 gates open... it's possible that he might last at least 5 minutes since kyubi really doesn't give you a chance to get a clear opening for an attack. But that's just my theory.