Gau is not part of the storyline, as this is an optional battle. However, he must be defeated in order to get all 108 Stars of Destiny.
As A BossEdit
Gau always appears with four Demon Wolves. He only has one attack, a three-hit combo that does about 150 damage.
He is not terribly hard to defeat, but an unprepared team can be wiped out easily. Start off by first defeating all the Demon Wolves, preferably in one shot. This will allow the player to focus solely on Gau. Unite Attacks are best against him, as he doesn't have any major weaknesses to magic aside from the Rune of Punishment, which does a little more damage to him than other runes.