I dunno, there's literally only 1 source saying this, but apparently Gamera vs. Wyvern (red link until it's confirmed this is real) was gonna be Daiei's version of Destroy All Monsters. Here's the would-be article:
Gamera vs. Wyvern (ガメラ対双頭怪獣Ｗ, Gamera tai sōtō kaijū W?, lit. Gamera vs. Two-Headed Monster W) is an unmade 1972Daieifilm.
The Virasians and Zigra's race join forces when they meet up by chance in Terra to research the now-frozen and dead planet, and unleash an flock of Space Gyaos on Earth. Their united invasion is led by the new monster, Wyvern.
Paira aliens decide to help Earth by resurrecting all of Gamera's past Earthling foes (Barugon, Gyaos, and Jiger), as a giant whale and a horde of Nezura appear to help as well. The monsters assist Gamera against the hostile invaders, which leads to a final showdown with Wyvern itself.