During the days of the meltdown, in which the vast walls of ice surrounding Ice Valley were giving way to the tons of meltwater behind them, a boat formed from an immense piece of wood had been the target for the herds of animals that resided in the valley. Gustav took it upon himself to act as gatekeeper for the boat, deciding to issue his own "pre-boarding announcements".
A herd of animals consisting of a mammoth named Manny, a ground sloth named Sid and a Saber-Tooth Tiger named Diego had found Gustav, who had decided to only allow passengers with mates to board: none of the three had mates, prompting Gustav to inform them that they must standby travel: upon Diego asking what "standby travel" was, Gustav informed them that it meant that they would stand by while all on the boat traveled. At that, Manny confronted Gustav on not having a mate. Gustav replied that he did have power, and as such, whatever he said, was official, to the dismay of the other animals boarding the boat. Manny, defying Gustav's authority, called out that the boat would be boarding all animals, regardless of whether or not they had mates. At that, all the animals present stampeded onto the boat, to Gustav's annoyance, who called out that he was in control.
The waters had soon flooded everything, and all the animals aboard the boat, including Gustav, were swept out on board the boat as it was carried out on the waters. At the end of the flood, Gustav called for all the animals to leave the boat in an orderly fashion: instead, the herds of animals all stampeded out of the boat, flattening Gustav as they ran.
As self-declared gatekeeper for the boat at the end of Ice Valley, Gustav was filled with a sense of authority, deciding to allow certain creatures leeway into entering the boat, and relishing his power and self-imposed authority over others.