Giga Bowser is a giant, monstrous form of Bowser who is the primary antagonist in Super Smash Bros. Melee. He may be fought in the game under a few circumstances such as replacing Bowser as the boss of Adventure mode in Normal mode or harder and in several Event matches. He is probably one of the most powerful character in the game. His attacks are the same as Bowser's but are stronger and have side-effects such as the Whirling Fortress (Bowser's B+Up Move) can freeze your character. He's even the slowest character because his speed is double the slow speed of Bowser. He cannot be grabbed and thrown out of the battlefield.
In Super Smash Bros. Brawl, Bowser once again morphs into this colossal being and wreaks havoc as he did in Melee. This will only happen, however, when Bowser comes in possession of a Smash Ball. As Giga Bowser, he is invincible unless he happens to fall off of a stage.
"An even more imposing figure than the original King of the Koopas, Giga Bowser is roughly twice the size of his scaly, fire-breathing, spike-studded, Super Smash Bros. Melee counterpart. Predictable, this monstrous creature's offensive and defensive powers are a grade higher than those of regular Bowser. Good luck defeating this colossus!"
"Bowser, transformed into the terrible and brutal form first seen in Super Smash Bros. Melee. While in this form, he's invulnerable--he doesn't get stunned and cannot be budged. His appearance is so fierce, it's as if he doesn't even belong in the Smash Bros. universe. He cannot maintain this form for long, so dealing out damage efficiently is key."