Question: why gohan has a tail?
Answer: Gohan was born with a tail, but since Goku and Chichi still did not know what it was capable of doing, they let the boy grow up in the same way as his father did. Piccolo was the one who ripped off his tail after seeing him become Oozaru. As with Kakarotto, Gohan's tail was born again in the battle against Vegeta.