Gau appears an enemy in Final Fantasy VI. He appears on the Veldt whenever he has used the Leap command to leave the party.
Gau's stats vary depending on what level he's at along with what piece of equipment (shield, helmet, armor, and relic) he's equipped with before he left.
When Gau appears, any commands issued to the party are cancelled. The first time the player meets Gau in Sabin's scenario, using a Dry Meat on him will begin a cutscene, after which Gau joins the party. All times after this when Gau appears after using Leap, he will rejoin the party after being left alone for a few seconds. Attacking him, even after earning his allegiance, causes him to flee and not rejoin. This will not affect future appearances by Gau, and he will continue to attempt to rejoin the party.
In the World of Ruin, wandering the Veldt in a party of less than four people will cause Gau to appear and rejoin the party. He will not appear if the party is full as there must be an empty slot for him to occupy when he rejoins.