Giga Bowser did not originate from fanon, and more information can be found on their respective wiki(s). You may read more about it on the following wiki(s):

Giga Bowser
Giga Bowser Art
Giga Bowser from Super Smash Bros. Melee
Gender Male
Species Koopa
Class Bowser transformation
Koopa Troop,
Ganondorf and Mewtwo
Main Weapon(s) Claws, Elemental breath, Spiky shell
Ability/ies Near invulnerability
Can breathe fire

Giga Bowser is an alternate form of Bowser introduced in the Super Smash Bros. series, appearing as a boss in Melee before becoming Bowser's Final Smash. Giga Bowser has elemental-based attacks, which gives a unique twist to Bowser's usually flame-based attacks, with explosive headbutts or spins that harness icy wind.

Fanon Appearances

Mushroom Kingdom Warriors

Giga Bowser appears as the main antagonist of Mushroom Kingdom Warriors. In the game's final story scenario ("Mario + Luigi"), Mario and Luigi accidentally knock Bowser into a button that transforms him into Giga Bowser and sends him into a rage, destroying Bowser's Kingdom before moving on towards the Mushroom Kingdom.