Reason why Gohan Stronger than Goku in Dragon Ball

Gohan was always meant to be stronger than Goku. You see this throughout the DBZ series at various points

Gohan

• Against Raditz when he powers up and is stronger than Raditz in an effort to save his dad

• During his training with Piccolo and he blows a chunk out of a mountain where piccolo states “one day, that kid will stronger than all of us”

• Against Nappa when he powered up and surprised everyone.

• When he takes on Vegeta (though Vegeta was weakened)

• When he and Krillin fight against Guldo and later against Recoom

• When he pushes back Frieza twice in two different forms

• Against Cell when he achieved SS2

• Against Majin Buu when he had the mystic powers.

Against Cell was a MAJOR turning point. Akira Toriyama planned to kill off Goku here. Which is why his goodbye here seemed sadder. The idea was for Goku to die and pass the mantle to Gohan who accepts. This is where the Bojack movie comes in and why the Bojack movie is so important. It was supposed to be the first movie and official introduction of Gohan as the new saviour and champion of earth. In this movie, Vegeta noted that he seemed even more powerful than he did against Cell and that he'd only get stronger.

But the fans threw a fit and after much grief, Toriyama came back and with the Buu saga and “revived” Goku. However, Toriyama still wanted to make Gohan the strongest. We see this when Buu absorbs the other Z-fighters but no one gives him nearly as substantial of a power boost as Gohan does. At this point, Gohan was officially stronger than any of the other Z-fighters. Including Goku.

You can confirm this with a simple deduction really.

Kid Buu is his base form.

Fat Buu is weaker than Kid Buu (but still incredibly powerful) because he absorbed the fat Kai and his kindness offset Buu's natural aggressive and apathetic behaviour.

Bad Buu (tall, skinny and grey) is stronger than Fat Buu. We see this when he beats him and absorbs him.

Super Buu (tall more muscular and pink) is his strongest form. We see this when he's able to take on the Z-fighters and take them out. But he runs into a snag against Gohan. But due to Gohan's caring nature, and lack of fighting experience, he loses to Buu and gets absorbed also. Resulting in a Buu so powerful, the Z-fighters NEED Vegito to stand a chance. Who absolutely overpowers Buu.

When Buu is separated from all the other influences, the only thing left is his base form. He was “getting stronger” (as noted by Goku) because he was expecting him to get back to the weaker fat form. But instead, he reverts back to Kid Buu. This is important because Goku and Vegeta can hold their own against this Buu. At least for a time. But SS3 Goku wasn't enough to beat Super Buu. Therefore we can conclude that Kid Buu is weaker than Super Buu, but Gohan was clearly stronger than Super Buu. Making Gohan (at this point) the strongest Z-fighter.

But again (as you'd expect) fans were less receiving of Gohan's new powers and so, they stuck with Gohan’s caring nature and his scholarly side. Resulting in him being depowered over the years.

In the latest series, Dragonball Super, the status quo leans heavily in Goku and Vegeta's favour. With Gohan being a distant 4th (behind Goku, Vegeta, and Future Trunks). But supposedly they (the writers) have something in mind for Gohan. Maybe we'll see him come back with something new. Or with a deeper understanding of his Mystic Powers.

In the world of dragon ball z, whenever a child is born he/she is having a specific power level.

As you might have seen that Goku and Broly were born on the same day but Goku was having a very low power level but Broly was powerful than grownup Saiyans. So, it depends on Saiyan to Saiyan. You can see even Vegeta did not have a high power level when he was born.

Goku and Gohan

Coming to Gohan. The same argument can be applied. He was born with a large power level. Even though he was half Saiyan and having this high power level at a young age was quite unique. But we know Gohan was unique from the beginning.

While Raditz was fighting Goku, he noticed a very strong energy signature from the direction in which Gohan was standing. Gohan’s power level was much higher than anyone he had seen. From that, the scene we got to know that Gohan is strong.

But at that point, Gohan was not able to control his power. Years later he learnt to control his strength.

It was Gohan who was finally able to kill CELL and Gohan was also able to release his mystic form in Majin Buu saga.

It was within Gohan from the very beginning. To be something big. It was all in the character design.

And of course, Boo vs Mystic Gohan.

DBZ

There have many few incidents were Gohan himself finishes off the opponent in DBZ. Nevertheless, Gohan had at some points reached an energy level way beyond his father’s. Goku kills the opponent often using spirit bomb, but Gohan reaches the energy levels where he could have finished off the opponent using his own energy, but couldn’t due to some other reason.