Mamoru is a seemingly normal 9-year-old boy in every respect: He goes to school with other kids his age, has two loving parents, and leads a peaceful life. But when the evil mechanoid alien Pasder awakens in his underground lair, things change for the worse. Sending his evil Zonderian servants to turn unsuspecting humans into giant robot monsters, mankind's salvation lies squarely on the shoulders of the Gutsy Geoid Guard defence agency. Together with the cyborg Guy and the mechanical lion Galeon, Mamoru finds himself as the key to ridding Tokyo of the alien menace!
GEAR Fighter Dendoh and GaoGaiGar use the same premise of an alien coming to Earth with ultra-advanced technology and a warning of an invasion, subsequently leading to a secret organization being assembled with the sole purpose of battling that invasion.
Both series feature elementary school kids are protagonists. GGG has Mamoru and Dendoh has Hokuto and Ginga.
The main difference in temrs of handling the plot lies in the way characters are handled, mostly. GaoGaiGar will introduce many characters but not spend as much time developing each of them individually, whereas Dendoh has fewer characters with more development. Both have merits, as GaoGaiGar is always one-upping the awesomeness factor, whereas Dendoh is more playing on growth and less on over the top awesomeness, though both series do feature some of both raw awesomeness and growth.
Also, both have pretty memorable stock footage.