When the Buu Saga began, Gohan had taken seven years off from training and it showed, as he was significantly weaker than before. So far in Dragon Ball Super, Gohan has suffered a similar fate, as a lack of training has led to multiple characters surpassing him in strength. As one of the principle Dragon Ball characters since his introduction in Dragon Ball Z, Gohan needs to reclaim his former glory and become a valuable member of the team again.
As a half Saiyan with more potential than perhaps any other character, Gohan has always been an important part of the series' major battles. His rage helped in the fights against Raditz, Nappa, and Vegeta. Against Frieza, Gohan was one of the main reasons the Z Fighters were able to hang on until Goku arrived, as his rage again gave him enough of a boost to take on second and later third form Frieza.
Gohan went from one of the heroes to the primary hero at the end of the Cell Saga. He was the only one strong enough to handle Cell. After years of waiting, he finally fulfilled his enormous potential, which made what came next so disappointing.
Gohan stopped training to focus on his studies. This led to him getting outclassed in the Buu Saga, until the Supreme Kai helped him unlock his potential and become Mystic Gohan. As Mystic Gohan, he was easily the strongest Z Fighter.
In Dragon Ball Super, Gohan has once again slacked off from his training. He went from being the most powerful Z Fighter by a wide margin, to being significantly weaker than Goku and Vegeta. He wasn't even able to slow down first form Frieza.
For the sake of the series, Gohan needs to start training again. With Vegeta and Goku frequently taking off for their own adventures, the planet needs a protector, and Gohan is the only one capable of reaching that level of power.
He also brings a valuable perspective to battles, but he can't contribute if he isn't powerful enough. Vegeta is arrogant and allows himself to get frustrated easily, whereas Goku is so relaxed that he can be caught off guard. Gohan, although far from the perfect fighter, doesn't have those same issues. He has a wealth of martial arts experience from his training with Piccolo and Goku. If he reaches Goku and Vegeta's level of power, he could help them in fights with more powerful foes.
Gohan was once seen as the fighter who would take over for Goku as Earth's protector. He was one of the most powerful characters for a large part of Dragon Ball Z. Seeing him become almost useless in Dragon Ball Super is a shame, as he still has the potential to be so much more. He may prefer studying, but as one of the most powerful fighters in the universe, Gohan has a duty to himself and to his planet. He needs to start training again so Dragon Ball Super doesn't revolve around only Goku and Vegeta.