What Could Have Been: There were no less than four different concepts pitched before Nikkatsu settled on Gappa:
The first was called Giant Monster Gigant, about a giant alien spider that came to Earth.
The second was called Giant Squid Monster Arkitius, about a giant squid that fights an Axis ship during WWII.
Then there was Giant Monster Momonra, which involved a flying squirrel Kaiju.
Lastly there was Reigon: Devil of the Seabed, where a stingray Kaiju battles an Iguana Kaiju.
Nikkatsu had briefly thought about making a new Gappa movie in the mid-90s after being impressed with the success of the Heisei Godzilla and Gamera films. While it never got made, somehow it spiraled into a long debunked rumor that the planned film was to be a crossover with Shochiku's Guilala despite the two companies never even discussing the idea.
Genral Trivia
Despite having a very similar plot to Gorgo the script writers for Gappa had never heard of the the film until after the fact.
