Films appeared in:
Gamera 3: The revenge of Iris the evil god / incomplete struggle
The reason Nightmare Gamera wasn't included with the Gamera info page was because he isn't real... so then just what is Nightmare Gamera???
Nightmare Gamera is the image Ayana Hirasaka has of Gamera in her dreams. He only appeared in Gamera 3 for a little less than 1 minute. In 1995, Ayana and her family were in Tokyo while Gamera and Gyaos fought. Her father put her in a van, while he went up to get Ayana's mother and cat, named Iris (No, that's not the thing that get's revenge in Gamera 3) so that they can escape. Gamera accdently crushed Ayana's apartment building, while her father, mother, and cat were still inside.
( If you want to know what part this happens durring the battle (From Gamera 1it's when Gyaos slices Gamera's cheek with it's super sonic beam, causing Gamera to drop down from the sky. Then, when Gamera get's up and smashes that apartment building out of the way to fight Gyaos, that is the building Ayana's parents were in ). We can only assume Nightmare Gamera has the same abilites of Gamera, but we may never know, since he isn't real.