Films: Godzilla vs. Gigan (1972), Godzilla vs. Megalon (1973)
Alias: Gaigan, Borodan, Galien, Monster Zero, Geigan
Location: Civilized area
Height/Weight: 65 meters and 25,000 metric tons.
Summary: Easily one of the weirdest kaiju of the Showa series, Gigan is what happens when your 4-year old gets bored of his parrot and asks himself "what if I could make it cooler". Naturally, that would just mean attaching a bunch of pointy things to it, and voila, you have this thing.
History: A cyborg beast crafted by the Nebula-born cockroach people from the planet Space Hunter M (whew!), Gigan has been their trump card for some time. When they inevitably found Earth, he was called to action once more, with King Ghidorah assisting no less. Then Godzilla and Anguirus showed up.
Notable Kills: Nothing special.
Final Fate: As usual, Godzilla drives the alien invaders away. Gigan did return when the Nebulans allied with the Seatopians to assist Megalon, but it ended the same way.
Powers/Abilities: Gigan can fly, has a rotary buzzsaw on his belly, and SUPPOSELY he can fire a laser from his eye, but he never does.
Weakness: Tends to be the first to retreat once monster battles stop turning in his favor.
Scariness Factor: 4-Gigan holds the dubious honor of being the first monster to make his opponents bleed. A lot. Seriously, it's pretty jarring to see that much gore in what is often considered the Dork Age of the Godzilla series. Aside from that, he's pretty okay design wise. Give it more than 3 decades later though, and he'll look MUCH more imposing.
Trivia: -Gigan, like Godzilla and King Ghidorah, made an appearance in the Ultraman-esque "Zone Fighter" series back then. In that, he needed parachutes to control his flight, and was obliterated by the titular hero. Then again, the canon status of that series with Godzilla is debatable.
-In the Japanese action figure-based TV spin-off series, "Godzilla Island", Gigan actually turns out to be a good guy, helping Godzilla and friends after being defeated the first time. Also, he wanted to be a samurai.