Just the fact that Gai was able to match with Kisame is pretty damn good. Even killer bee, not without the help of Samehada or his bro, Raikage was able to beat him. Gai, a mere jonin was able to keep par. Also, it seems the 8th gate, is located where the heart is. So maybe you don't necessarily die if you use the last gate, but just have a higher chance of dying. Opening that last gate is most likely going to cause his heart to beat rapidly and prolonged use if it, will cause him to die. However, if he were to only use it for an instant, overpressuring is heart wouldn't cause death.