Gamera is (an avatar of) The Turtle.
The turtle is, after all, the natural enemy of Pennywise/IT. IT is a monster that preys on children, while Gamera is the friend of all children.- Whether or not this theory holds any water, you can't argue that the idea of Gamera fighting against a Kaiju expy of Pennywise wouldn't be awesome.
Eiichi and his Sister's parents were killed by Barugon during his raid in Osaka.
This is why the parents are completely absent from Gamera vs. Gyaos and the two kids are living with their grandparents. It's also why Eiichi loves Gamera— he felt the giant turtle got Barugon back for his parents. Iris was Evil All Along.
The Gamera 2.5 manga (which is sadly unavailable outside Japan) reveals that Iris was one of the first attempts created to stop the Gyaos. Having Gyaos DNA in him probably meant he turned against his creators at some point. And though it's never confirmed, I think he was simply using Ayana to get more power, the moment where he bombards her with images of the transpired events and her hand in all of it doesn't seem likely to be the actions of a creature under the strict control of someone.Plus his name in the kanji representation means "evil god" or "false god."
Had an official fourth Heisei Gamera Film been made, it would NOT have been an Immediate Sequel to Revenge of Iris.
Kaneko said that the idea behind the Bolivian Army Ending of the third Gamera movie was that you’re supposed to believe that Gamera would prevail, so having the next movie pick up right where the previous one left off would undercut that message. Instead, the enemy Gamera would face would either be a brand new Kaiju or a classic Kaiju from the Showa Era.