If Gau is no longer on the battlefield when its last ability resolves, use its power as it last existed on the battlefield to determine how much damage the ability deals.

Gau's last ability checks at the moment it would trigger to see if a card left your graveyard this turn. If none did, the ability won't trigger at all. Once your end step begins, it's too late to make a card leave your graveyard in order to cause this ability to trigger.