Gotenks is the powerful result of Trunks and Goten using the Fusion Dance Technique. They used the fusion dance technique fighting against Majin Buu in the DBZ Majin Buu saga. He appeared a bit at the end of DBZ playing bath fight with Goku and in Yo! Goku and His Friends Return special he appeared fighting against the aliens. He's not in GT, Goten said to Goku if him and Trunks fuse they will fight Bebi but Goku said it's no use.