kisame heals any damage he does like when he healed from bees lariat. and no hes not the counter he just kept fighting weakened kisame
In order for kisame to heal himself, he first needs to absorb the opponents chakra. Its gonna be pretty hard to absorb the chakra of someone of Gai's speed who also only uses taijutsu since he would have samehada to absorb directly from his body, which doesnt allow it to absorb as much chakra as it would of it were outside of the body like Killer Bee's cloak.
Also, each time they fought, Gai
destroyed Kisame like the second time, despite him not having full chakra, and the second time, since Kisame also had the terrain advantage and Gai was not even trying to kill him with Hirudora, but to widen his scope of attack to destroy all of the underwater sharks at once. Kisame just thought it was going to be a chakra missle, so he tried to counter it with the large shark missle to absorb it and destroy Gai at the same time to keep him from destroying the other sharks.
when was the third time? and come to me when gai has a counter for both fused samekisame, and the massive lake dome jutsu
The third time was the final time that they faced. They first met in Konoha when Gai appeared to support Kurenai, Asuma, and Kakashi and Kisame was told by Itachi not to underestimate Gai's power. The second time was in near the akatsuki hideout with Team Gai
Gai has already used the 6th gate to repel kisame's instant lake jutsu when he was pushed down to the very bottom and was about to be eaten by sharks, so that alone nullifies Kisames advantage in the water, and Gai could then proceed to destroy any remaining water with morning peacock, forcing Kisame to stay on land, and then speedblitz him like Lee did Gaara in the chuunin exams. Kisame won't be able to react either by himself or with Samehada to absorb his chakra, and since he doesnt have any sort of defense for Gai's taijutsu like Gaara did, Kisame will die.
so gates counters kisames giant ass lake that pressured even bee? how is gai a bad matchup if hes only ever fought weakened kisame?
It would as I explained earlier, Gai has already counterd a giant lake before, but the thing is, Kisame requires a large amount of chakra for it since he only did it after he absorbed a substantial amount of Killer Bee's chakra and the bijuu cloak. If he were to do it himself with his own chakra, he would waste a great amount of chakra creating the lake. Not only that, but Killer Bee had to worry about the giant racoon and his enka master, who had both been caught up in the jutsu so he couldn't fight at full power himself.
I have also explained how each time that they fought how Gai destroyed him each time and even gave a scenario where he is at full power with samehada and even a fused kisamehada, yet would still defeat him. Gai is simply Kisame's greatest counter with his taijustu only attacks and speed+strength advantage.