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.
The Reason: The only advantages Gamera has over Godzilla is flight and intelligence, but that shouldn't be a problem because Godzilla has fought foes that has flown as fast as Gamera and were more intelligent, other than that. Godzilla outclasses Gamera in everything else. Strength, durability you name it. Godzilla walks through military firepower like it is nothing, can you guess what has floored Gamera. Yep that's right, military firepower. Then there is Godzilla's regenerative abilities. Yes, Gamera has it too, but in order to regenerate he needs to go into a state of hibernation, he can't regenerate while fighting, what makes up for it though is his extreme pain tolerance, having been amputated, and impaled, yet still continues to fight like nothing happens. Unfortunity I don't think that'd be enough though. Gamera's only shot in winning is the Mana Blast, but that takes time to charge up and knowing Godzilla. He would not give Gamera the chance to charge.
Then if we don't ignore weight. Godzilla weighs 500 times the size Gamera does. For example.
If Gamera were to use his spinning shell flight as an attack (like a frisbee), I'm sure that would knock some wind out of Godzilla's lungs. And if Gamera had time to charge his belly blast or whatever it's called and use it on Godzilla, then long live the king!
And even in intelligence should really be even. Each incarnation of Godzilla showed a distinct learning curve, take SokogekiGoji (GMK), any one attack never worked twice on him, Mothra sneaked up behind him and hit him with her shotgun blast thing, next time she tried that Godzilla hit her with his tail then evaporated her.