I don't remember Dagarah being all that fast. Oh, sure, in the water he could probably swim circles around Gamera, but on land I think Gamera's got the advantage. As for the air... from what I can recall about Dagarah's flight speed, I'd say Gamera is a bit faster.
As for their weapons... while Gamera's plasma fireballs are highly explosive, they are his only weapons. Dagarah can fire that lightning from his mouth, as well as shoot off lightning and explosive blasts from his shoulders, beams from his back, rotating energy discs... hmm...
The Mana Beam would, of course, obliterate Dagarah. However, Dagarah is a beam-happy Heisei kaiju and would fire about two dozen energy blasts into Gamera's open shell before he could fire.
The whirlpool would probably be a less-severe version of the Legion soldiers to Gamera. Though they wouldn't cut into him, they would weaken him...
I appologize if I'm stating the obvious here, I'm having difficulty deciding who would win.
Another factor is an underwater fight. If this were to happen, we know how Dagarah fares underwater. How well would Gamera function under the ocean's surface? This might be the deciding factor.
That aside, if Gamera can destroy Dagarah's shoulder cannons and somehow avoid the whirlpool, he might have a chance. But if he can't (and since there's no way Dagarah will give him a chance to use the Mana Beam)... I have to give this one to Dagarah. It took Rainbow/Aqua Mothra to defeat Dagarah, and I doubt Gamera has that much power.
Who knows, though. Maybe Gamera could form a Berem Fist or something.
