As Kisame exited the waterfall of truth, Gai realizes that it is not his real inner self. Gai finally recognize the face, and also Bee recognize that he is one of the Akatsuki.
Semehada suddenly breaks with Kisame and goes to Bee with thinking that Semehada is choosing him over Kisame until he finally realize it that it is after its chakra. Gai faced Kisame and opened his sixth gate of his eight celestial gates to be able to pursue and fight Kisame. Kisame rushes to get out of the island to relay the info regarding the training and whereabouts of the two remaining Jinchukiri.
Since Kisame is a water type he was able to use his attacks and jutsu. Gai finally cornered Kisame. Kisame uses his greatest jutsu to kill Gai in one blow...on the other hand Gai opened his seventh gate and he is betting his life again to beat Kisame.