In Melee
In Melee, Giant Bowser appears at the end of adventure mode, where you fight him at Final Destination. He may or may not be the last boss. If he is regular colored, he is the last boss. If he is black colored, the last boss may be Giga Bowser(if the time limit is not exceeded and the playerdo not continue).
In Brawl
He appears in an event match.