The Gong Forest is an island very close to the start of the Grand Line, probably at the same distance as Little Garden.
Ages ago, the giant Fogg stranded upon the deserted island. He found himself at the border of death since it was a desert back then. After being saved, Fogg cursed the land and left to never see it again not knowing that he had actually forgotten his pet monkeys. The monkeys breed and found an underground passage which was full of water and plants. The monkeys decided to live here but after some while, the Flying Menace Pirates arrived there to suck all live out of the place. Upon finding the passage, their leader, Edward Falcone felt sorry for the monkeys and decided to actually help them bringing down the ceiling of the passage making sunlight come in and make the trees grow higher. He promised that one day he would come back and take all the live of the island though. The trees eventually became so big that they got out of the passage and made it to earth level. The great elder Kakuma arrived one day to the island and seeing all the vegetation saw this as a great place to live after the Flying Menace Pirates sunk down their island. He then started a village and named the place the Gong Forest due to the large tree in the middle of the island in the form of a Chinese Gong.