I am a big fan of Japanese monster movies and grew up watching Godzilla, but also took on a handful of Gamera films. Overall, these movies are treasures from my childhood and even to this day. However, as with all movie trilogies, there are a few that stick out in a negative way including this one.
Some of those flicks are so cheesy and bad, it's good. This Gamera flick, about the flying turtle trying to save Japan from Triceratrops-like monster Giger awoken at the Expo '70, doesn't reach that scale. The plot and story are very plain and suffers from a lack of excitement, tension and thrills - nothing to captivate the audience. The cast of characters (adult) are not memorable and didn't really contribute a whole lot to the story - they were just thrown in to be bystanders, letting the main three kid characters take center stage, this time trying to save Gamera (shown in a very campy scene where they board a submarine and enter Gamera's body to detect the source of his illness in the movie). Every Gamera movie except Gamera vs. Barugon in the first series featured at least one kid as the main character and a good handful of them were pretty annoying. Lastly, the cheap-looking monster suits continued in this film; the monsters didn't show any emotions or characteristics like the creatures you would see in Godzilla films. They just look like stiff, moving museum models.
Again, most of the Gamera films are mostly for children and they would be good to use to introduce them to the world of Japanese kaiju. Other than that, most, including this one, were very campy and cheesy with bad acting, bad special effects and non-captivating stories. If you enjoyed these films as a kid, you would probably grow out of them - the enjoyment factor is not as everlasting as the Godzilla movies or the later Gamera films in my opinion.