I am going for Godzilla bitch survived a black hole a dead on hit from a meteor without even a scratch and the presence of Godzilla can turn half of Tokyo unlivable due to the radiation he admits plus Godzilla can breathe in outer space and he can fly to do to his atomic breath plus godzilla's red atomic breath can destroy everything and it could possibly pierce gamera's shell and gamera has self-destruct move so godzilla would eat it up godzilla is literally a giant tank.
Well, one version of Godzilla fought the Avengers & Fantastic Four and that Godzilla wasn't even his strongest incarnation. Also, the enemies Godzilla has fought and defeated are in a different league than Gamera's enemies, so I say the King keeps his title against the Guardian.
King Ghidorah murderstomps Gyaos. King Ghidorah is roughly as strong as Godzilla, Godzilla's major advantage being his regenerative powers and huge physical strength and King Ghidorah's major advantage is the ability to fly and his 3 heads giving him the option to use one of them to attack physically while others fire Gravity Beams You could say his size is also an advantage but Godzilla makes any size completely irrelevant. Gyaos are usually more than one and Gamera tends to 1-2 shot them... not even a challenge for King Ghidorah.
Super Gyaos would still be very easy for the King of Terror. Irys might be a bit harder but I still don't see KG lose to it. King Ghidorah has had one and even two of its heads cut off more than once and still managed to fight on. Such vigor is dangerous.