Akira Toriyama's perennially popular manga/anime franchise Dragon Ball became a generational story by the start of Dragon Ball Z, with protagonist Goku and his wife Chi-Chi welcoming a son in Gohan. Revealed to be a hybrid of Saiyan and human genetics, Gohan demonstrated an immense amount of combat power from an early age, shocking his father during a battle against the evil Raditz. While DBZ would introduce another human-Saiyan hybrid in Trunks -- the son of Vegeta and Bulma -- Gohan would always have a significant edge as the strongest mixed-race Super Saiyan in the franchise to date.
The future timeline's Trunks was trained by an adult Gohan to battle androids that had devastated Earth and killed most of the Z Fighters after Goku succumbed to a terminal heart virus. While this timeline's Trunks showed potential, it wasn't until he was a teenager and fueled by grief over Gohan's death that he would become a Super Saiyan for the first time. Still no match for the androids himself, Future Trunks traveled to the prime DBZ timeline to save Goku from the virus while training to become even stronger. Despite this, the prime timeline Gohan became a Super Saiyan 2 roughly around the same age as Future Trunks had achieved his base Super Saiyan form, with Gohan being the most powerful Z Fighter for a time.
Seeing the power of mixed-race Saiyans firsthand, the prime timeline Vegeta trained his Trunks ever since the boy could walk to one day become just as strong as Gohan as part of his obsessive quest to surpass Goku's legacy. While this Trunks was able to transform into a Super Saiyan years earlier than his divergent timeline counterpart, Gohan consistently outpaced him as he reached new power levels, as seen towards the end of DBZ into the sequel anime series Dragon Ball Super.
What is the reason behind this gap between Trunks and Gohan's ability? Is it time spent training, or does it come down to genetics -- factoring in their respective mothers, Bulma and Chi-Chi. After all, Chi-Chi's background included martial arts training, especially in Dragon Ball, while Bulma took up more technological pursuits. Even in the series' more comical moments where Chi-Chi's rage comes out, she's often shown as strong in her own right. Still, while their mothers undoubtedly contributed to Gohan and Trunks' growth and potential, there is a significant factor that edges Gohan ahead.
The big reason behind Gohan becoming stronger than Trunks is he had two separate power boosts outside of conventional training that gave him a crucial edge. While on Namek, Gohan and Krillin both had their latent powers awakened by the Namekian elder Guru to aid them in the fight against Frieza. Years later, as an adult, Gohan would undergo a lengthy process by the Elder Kai, which similarly raised his combat potential -- unlocking the Mystic Gohan form that was initially depicted as being on par with Super Saiyan 3.
By the time of Dragon Ball Super, Gohan revealed to his father that his mystic transformation was separate from the traditional Super Saiyan progression, with untold levels of power waiting to be discovered. With that in mind, Mystic Gohan was able to spar with Goku while the latter was transformed into a Super Saiyan Blue, showcasing just how much stronger Gohan had become since the end of DBZ.
Trunks is still one of the most formidable Z Fighters in the franchise, reaching Super Saiyan 2 by his return in Dragon Ball Super and achieving a maddened Super Saiyan form in his duel against merged Zamasu, but Gohan remains significantly stronger. The two processes that boosted Gohan not only catapulted him above the other mixed-race Super Saiyans but set him on a completely different path to power altogether. And with Gohan back into his training to become his own kind of warrior, it doesn't appear that Trunks will catch up to the older Saiyan anytime soon.