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.
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.