One Piece Chapter 22, page 17-19: YOU ARE A RARE AND PRECIOUS ANIMAL TOO

Gaimon begins his flashback from twenty years ago. His pirate crew had landed on the island after finding a map indicating treasure was hidden there. However, after three weeks of searching, the crew of two hundred men found nothing but a single empty, broken treasure chest. The pirate captain, seen from behind wearing a dark coat and bicorne hat, declares the search over and orders everyone back to the ship. A younger Gaimon, wearing a striped shirt, reluctantly prepares to leave.

In the flashback, young Gaimon realizes that no one has searched the top of the tall, steep rock hill in the center of the island. He climbs up the cliff and is stunned to find five treasure chests sitting at the top. Excitedly, he turns to call out to his crew, but slips and falls off the cliff, landing directly inside an empty wooden chest below.

By the time young Gaimon woke up, his crew had already sailed away, leaving him behind. Stuck inside the chest, he crawled to the shore, crying out for them to return. Realizing he was stranded, he decided the treasure on the hill was all his. However, he found himself unable to climb the steep hill while trapped in the chest. Over the next twenty years, he used his gun and the "forest judgement" to scare away any other pirates who came looking for the treasure.