During the planning stages the movie was originally titled King Ghidorah's Great Counterattack, which would feature Godzilla, Varan, and Rodan fighting King Ghidorah and a monster named Mogu. The script was reworked and retitled as Godzilla vs The Space Monsters - Earth Defense Order. The movie would have starred King Ghidorah, Gigan, and Megalon and the idea was scrapped as well. Although, the monster Megalon would be in Godzilla vs Megalon a year later. The script was rewritten again and finally became Godzilla vs Gigan.
This would be Haruo Nakajima's last movie to play Godzilla before he retired.
When a comic artist was hired for a concept artist position for a peace-theme amusement park, he and his friends discover that his boss is actually an alien sent to invade Earth. After summoning Ghidorah and Gigan to attack Japan, Godzilla and Anguirus intervene to put a stop on their attack.
Godzilla is at a point now where he is now looked at as a superhero more than an invading menace. Unlike the last movie, Godzilla vs. Hedorah, this is actually more child friendly. I have no clue what Gigan is suppose to be, but I heard that he is some sort of cyborg chicken. Some stock footage is used for this movie to cut costs on the budget, but it isn't over used. The only difference between the Japanese version and the American version is that in the American version they use annoying scratchy sounds and voices for when Godzilla and Anguirus talk, but the Japanese version uses speech bubbles.
One of the issues that I have with stories in these movies is that I am not too invested in them nor care about the characters. Godzilla vs Gigan actually got me some what invested in the movie's story and it didn't feel boring at all.
Godzilla vs. Gigan is a child friendly film that the whole family can watch together. I say check it out.