I saw this when I was a kid growing up in Michigan, watching 'monster week' on the 4:30 movie on channel 7. Just recently I purchased a DVD release from Diamond Entertainment called 'Japanesse Monster Movies' that contains four Gamera movies altogether. I used to like this particular film so I was interested in seeing this again.
For starters, a lot of the sets and I believe even some of the props and costumes have been taken directly from 'Gamera vs Guiron'. But at least 45 percent of this film is composed of stock footage from older films. Perhaps the funniest scene in the movie is when they show footage from the original Gamera film- a black and white movie- and try to pass it off for something that is happening in the present, even though this film was shot in color.
The kids in this film take charge of the situation and try to help Gamera out when the adults are content to do little or nothing. There's a lot of Kiaju 'gore' in this film, a funny yet kind of cool alien invader and stupid, stupid henchmen who get their heads knocked off )!) before the mutate into other squid-like creatures.
This is actually less boring than the previous films. Since the aliens are studying the previous Gamera fights in order to defeat him, the film is full of fairly cool monster battles. The final battle against Viran is a good one!