Night Turtles is probably more likely. Actually, since Gai is all about youth, he'll probably use young turtles, so "Teenage Night Turtles" is probably more likely. On second thought, these are going to be fighting turtles, so maybe something along the lines of "Teenage Ninja Night Turtles." He may even add some random word in there like mutant, so "Teenage Mutant Ninja Night Turtles" for the final combo.