Giga Bowser is a character in the Gamecube game Super Smash Bros Melee. He looks fairly similar to normal Bowser, except being around three times the size, and looking far more menacing. Giga Bowser's moves are also pretty much the same as regular Bowser's, just far more depicting, as well as having a far greater range (due to his increased size).
In Super Smash Bros Melee, Giga Bowser can be faced at two different times. The first of these is in adventure mode. To do this, you must go through adventure mode on either Normal difficulty or higher, and reach the final level (the fight with regular Bowser) in less than 18 minutes. This can be done with any character (even secret characters), but you can not use a continue at any time (you can lose lives, but you can not reach the 'gameover' screen). After defeating Bowser (again, without using a continue), you will see his trophy transform. Now comes the tricky part – the final fight. If you manage to beat him, you'll be rewarded with his trophy. Note that this is one of the hardest of all to get in the game, and is no small achievement. The fight alone may take you multiple attempts, but do not give up – just try an learn his patterns and moves, and success will not be far away.
Super Smash Bros Melee Giga Bowser can also be cooked in Event mode. After clearing all 50 of the first events, a special event becomes unlocked – Event 51. In this event you must fight Giga Bowser, Mewtwo, and Ganondorf, all with three stock (lives), on the Final Destination level. This is by far the hardest event of them all, as you too only start the level with three stock, and there is nowhere to hide.
Sadly, Bowser is not a playable character, and you can only fight him in the two points of the game that I have just mentioned. However, in Super Smash Bros Brawl (on the Nintendo Wii), he becomes a semi-playable character. When playing as regular Bowser during a match, if you manage to unleash his 'Final Smash' move, you'll be transformed into Giga Bowser for a small length of time.