Dragon Ball's titular wish-granting orbs possess the capacity to resurrect the dead. While the Eternal Dragon Shenron's first wish in the series was to bestow Oolong with panties, nearly every subsequent wish has involved reviving the departed in some form -- whether it's Upa's father Bora, the many civilians who have died at the hands of the series' villains, or even bringing back Goku himself. The Dragon Balls have been responsible for raising the dead countless times over the course of the franchise.
This may leave some fans, especially those who started with Dragon Ball Z, scratching their heads as to why Son Goku has never used them to bring his beloved Grandpa, Son Gohan, back from the dead. After all, the only stipulation when it comes to whether or not a person can be brought back is if they died of natural causes, which Grandpa Gohan definitely didn't.
Goku's adoptive guardian and mentor perished by Goku's hand (or under his foot) before the events of the series. When the orange-clad Saiyan first transformed into a Great Ape as a boy, he unknowingly killed Grandpa Gohan in the process. When he finally makes the discovery during the Saiyan Saga, Goku seems horrified, vowing to apologize to his grandfather next they meet. Fans who are only familiar with Z-onwards might beg the question -- why not ask Shenron to resurrect him?
It's a matter answered definitively in Episode 76 of the original Dragon Ball, ''True Colors of the Masked Man''. After a gauntlet of matches against Fortuneteller Baba's entourage of undead fighters, Goku finds himself up against a mysterious masked fighter with a halo who seems familiar with the young Saiyan's fighting style, even using the Kamehameha at one point.
This fighter is revealed to be none other than Goku's departed grandpa, back from Other World for a day courtesy of Baba's magic. After an emotional reunion, Goku's friend Upa asks the question at hand -- why not revive Gohan permanently with the Dragon Balls? Grandpa Gohan declines, stating that he actively enjoys the afterlife. Content to leave Goku after seeing how much his adoptive grandson has grown and matured, Gohan peacefully departs.
It's a carefree sentiment that makes it clear where Goku gets his personality from. After all, when Goku himself perishes, he takes a similarly blasé view of death. Once he's confident that his own son can take care of things on Earth, Goku seems perfectly happy to stay dead, ready to pursue new adventures in the Other World like his Grandpa before him.
While Goku himself would eventually come back, it seems unlikely Grandpa Gohan will ever return to the land of the living. Regardless, his spirit lives on -- both literally in Other World, and figuratively on Earth. Goku's firstborn Gohan was named after him, and his keepsake of the Four-Star Dragon Ball remains an important heirloom to the Son family. Even decades after Grandpa Gohan's death, his importance to Goku remains steadfast.
Recent developments in Dragon Ball Super may posit that Goku gets his benevolent nature from his biological father, Bardock, but Grandpa Gohan's effect on the series' protagonist can't be overstated. For all of Goku's flaws, he's remained a kind-hearted and honorable character since the very first episode, and that's thanks entirely due to his upbringing at the hands of his adoptive grandfather. It's those values that ensure Grandpa Gohan will live on forever, Dragon Balls or no Dragon Balls.