Grandpa Gohan (孫悟飯, Son Gohan; also referred to as just Gohan) is the adoptive Human grandfather of Goku. He is often referred to as "Grandpa" Gohan to separate him from the more popular Gohan character, named after him.
Grandpa Gohan was a loving and caring man who became the adoptive grandfather of Goku when he found him near the landing site of his space pod when he was a baby. When Goku first landed on Earth, his mission was to, in time, destroy the entire human race and sell the planet to another race. Being a Saiyan, who are known to be very aggressive and short-tempered, Goku was very uncooperative at first (in the TV special Dragon Ball Z: Bardock - The Father of Goku, he simply laughs as Grandpa Gohan found him in the wilderness).