How do you become giga bowser in Super Smash Bros melee

The only way to become Giga Bowser in Melee is by using an action replay for the GCN. Otherwise you'll encounter Giga Bowser if you get through Adventure mode in 18 minutes or less. You'll also get the Giga Bowser Trophy if you succeed in not using a continue on normal or hard.

The only way to use Giga Bowser legitimately is by getting Super Smash Bros Brawl and using Bowser's final smash.