Just to put things into perspective, Gamera being in this is about as likely as Batman being in the next Avengers movie. Gamera was created by Daiei to compete with Toho's Godzilla (Daiei is no more though, but their IPs got bought by Kadokawa in 2003). I am not sure about domestic box office, but obviously it did not leave a very lasting mark outside Japan. Unless you remember fondly Barugon, Gyaos, Viras, Guiron, Jiger, Zigra or Iris.
That being said, is this film related to the Kong: Skull Island thing? No cast and crew seems in common, but both are Legendary Pictures films....