Filmed at the real Expo '70 World Fair in Osaka, Japan.
Interestingly, during Expo 70 there was a stage show featuring Gamera alongside his box office rival Godzilla. This turned out to be the closest the two ever got to having the long awaited encounter between each other. No footage, except for a few seconds of Godzilla walking with Gyaos, of the even exist.
According to director Yuasa, the idea of Jiger injecting its offspring in Gamera was done as a cost cutting measure. The Gamera films from Daiei were given smaller budget with each subsequent film. Yuasa managed to convince Daiei to provide some additional funds for miniatures.
One notorious scene is the documentary footage of a lump on an elephant's trunk being cut open to reveal a parasitic worm infection. This was filmed for the movie with cooperation from the Tama Zoological Park. Makeup and a prop was applied to the elephant's trunk and filled with roundworms from a pig. Yuasa joked that he himself found it creepy which is why it was only shown in black & white.
Will be an experiment in Season 13 of Mystery Science Theater 3000