Honestly, this is much more Barugon's movie than Gamera's (it seems that they wrote themselves in a corner and couldn't find out a way to get rid of Barugon, so they shoe-horned Gamera in to kill him), but nonetheless, it still has Gamera in it, so it's considered to be a Gamera movie. But I digress, let's get on with the plot:
Generally, Gamera's still in a rocket... but it gets by a meteorite, which instantly destroys it, thus freeing Gamera. He flies back to the Earth and pissed as ever, starts destroying Osaka... but that's not important! Meanwhile, as we go through many subplots, there seems to be some kind of gem thingy (which is really a freaky-ass egg) that somehow wakes up Barugon (who certainly isn't confused with Godzilla's Baragon). And what do ya' know, someone ends up picking it up. Whoops. Barugon's powers are the ability to stretch his tongue out very far (it can also spray a mist that can freeze things) and shoot a rainbow out of his back ("Look, a Skittles commercial!"). Sounds pretty stupid, huh? But believe it or not, he's actually more intimidating than most of Gamera's other opponents. But back to business, Barugon arrives in Osaka and ends up freezing Gamera with his tongue, thus disposing of him for most of the movie. As Barugon wreaks havoc, the human populace attempt several tactics to get rid of Barugon. They all involve luring him into the water, however, since he can easily drown. They end up using both his egg and a diamond (seeing as he likes shiny things) to lure him into the ocean, but neither plan works. Eventually, though, Gamera ends up thawing out of the ice and after a brief battle, ends up holding Barugon down in the water, drowning him. Gamera then flies away... and then the humans don't attack the very monster that started destroying Osaka first and damn near obliterated Tokyo a year prior, despite the fact that he's perfectly vulnerable. Man, the Japanese army is USELESS. Get some more Americans down there.