Gamera is depicted as a giant, fire-breathing, prehistoric turtle monster, mutated by exposure to nuclear weapons. In the first film, Gamera is portrayed as aggressive and destructive, though he also saves a child. As the films progressed, Gamera took on a more benevolent role, becoming a protector of humanity, and especially children, from extraterrestrial races and other giant monsters.

Last summer. Friends. And Kaiju. 
In the summer of 1989, sixth graders Boko, Joe, and Junichi are on their last summer vacation as elementary school students. 
As if to cut through their daily routine, a monster named Gyaos suddenly attacks Tokyo. The four are petrified in midst of a town turned to ruins. As Gyaos sets its sight on them, a huge turtle-shaped kaiju appears in front of the boys. Its name is Gamera. Despite its injuries, Gamera fights Gyaos with all its might. This would only mark the beginning of the boys' "Summer of Kaiju".
 

(Source: KADOKAWA)

Special episode of Odekake Kozame promoting GAMERA: Rebirth.

A billion years ago, there was a super-ancient civilization.
The ancient civilization which possessed high technology became instinct due to a terrifying plan led by a genius scientist.
How did the man grew up to be someone to destroy the world?
A spin-off manga which explains the birth of KAIJUs in "GAMERA -Rebirth-." 
Enjoy the battles of fearful KAIJUs!
 
(Source: BookWalker)

Special episode of Yoru wa Neko to Issho promoting GAMERA: Rebirth.