In 1965 the titanic mutant turtle burst forth on the silver screen in Japan, and later in 66 he showed up in slightly altered (and I'd argue better) form in the US. It was his only US theatrical release back then, the following six movies were released directly to television. Only one other movie bore the name Gamera, so you never knew if you would be getting a Gamera movie at times. Which was kind of fun back then. I really had no idea that Attack of the Monsters was going to be a Japanese monster movie.
The idea to use a giant prehistoric turtle is interesting. Gamera, however bears little resemblance to the Archelon, the 12' turtle (seen in much enlarged form in 1966's most excellent One Million Years, BC). Gamera can walk on two feet, has huge tusks and can breathe fire.
He also has the ability to fly. It comes in handy when rescuing various children or protecting the Earth from alien invaders. The newer Gamera movies added several more powers, usually at the very last minute of the movie. It's been several years since I've seen any of them, so you'll have to look yourself.