Once you have the Airship, you can go to the Veldt to search for one of your friends.

Prepare and Go to the Veldt

Go to the Airship and form a party of three people. Then land on the northeast continent on the map to find the Veldt. Fight battles until Gau appears. Just wait for a while, and Gau will join you. He can learn more Rages on the Veldt just like he did before.