Gamera vs. Guiron (1969) on Netflix

Plot summary

Through a telescope, Akio and Tom watch a flying saucer come down in a field nearby. Akio's mother calls the story childish nonsense. The next day the boys pedal to the site with Akio's younger sister, Tomoko, and Akio and Tom steal aboard the craft, which lifts off without warning and leaves Tomoko behind. It races into a field of asteroids, and Gamera appears and clears a path for it, but the ship, traveling near the speed of light, outruns him and lands on Terra, a world orbiting the Sun opposite Earth. There Barbella and Florbella, the planet's only inhabitants, say that Terra is growing colder, that Space Gyaos are overrunning it, and that a monster named Guiron is their last defense.