What would the outcome have been if the IJN had Gozilla AND Gamera on their side? 
Turtle meat is tasty sweet.
Does the U.S. get Captain America?
Godzilla may be able to shrug off tank shells, but hit him with the 16 inch guns, and he’s lizard meat.
Gamera? He’s a turtle! We’ll just walk around him.
Nothing. At this stage of the war, even the two can’t help Japan.
OK, I kid because I’m sure someone will accuse me of thinking that.
Unfortunately, I’ve never actually seen a movie starring either one, so I need some help.
How much can they actually withstand? Wiki says that Gamera can regenerate himself. True?
How intelligent are they? How much would they cooperate with the IJN? Would they be like trained dogs or more like zombie level?
Do the Allies have anything which would be effective?
The problem is that this is pretty late in the game. Had they showed up at Guadalcanal, they could have turned the battle and [del]kicked out[/del] ate? the Marines. That would have significantly changed events.
Going back to Pearl Harbor, they could have actually destroyed the tank farm, dry docks and submarine base, as well as smunched (technical term, IIRC from kindergarten) the tankers, cruisers, destroyers and the rest of the fleet. Then they could have done the same for San Diego which would have put the US in a real bind. Suing for peace would be a good idea.
However, by the Battle of Layte Bay, the US had so much equipment and so many ships and the Japanese were so beaten up, having the US forces wiped out at the Philippines would not have completely turned the entire war. It would set it back, but it would be recoverable.
How come Ultraman gets no love?
I had an Ultraman pen!
Well, it was actually one of those multi color pens. But in MY hands…!
The Japanese ate enough Americans on their own.
With Godzilla and Gamera, why would Japan need a navy?
For the beans.
Would Paul Bunyan, Old Stormalong, and Pecos Bill be available?
Which one would say, “When we are through with them, the only place Godzillian will be spoken is in hell!”?
Ah, gas warfare.