The first of the new generation of Saiyan hybrids, Gohan was the biological son of Goku and ChiChi. Gohan was introduced in Dragon Ball and his growth as a character is depicted in Dragon Ball Z.
Born and raised as a peaceful child, Gohan didn’t learn to fight from a young age like his father but certain circumstances forced him to undergo a harsh training regime. This was done under the guidance of Piccolo in order to build a formidable fighting force against an unknown threat. From that day on, Gohan has grown at an extraordinary rate and has become one of the strongest fighters in the universe.
10 Kid Gohan (Before Goku's death)
The first time we are formally introduced to Gohan in Dragon Ball Z is when he arrives at the Kame House to meet up with his friends. Gohan is still quite young and still retains his Saiyan tail at the time.
However, after being introduced to Goku's friends, he is suddenly captured and used as a hostage to lure out Goku by his uncle, Raditz. During the battle, after witnessing his father get brutally beaten up, Gohan unlocks some of his immense hidden potential power and propels himself head first into Raditz. The blow is strong enough to crack Raditz's armor and stagger him long enough for Goku and Piccolo to finish him off.
9 Kid Gohan (after the death of Goku)
After defeating Raditz, Piccolo is given a warning about the pair of Saiyans (Nappa and Vegeta) who will arrive in two years time. With Goku sacrificing himself to help defeat Raditz, Piccolo is forced to train Gohan so he can become stronger. He acted as a substitute for his father and put Gohan under intensive survival training.
His strength grows exponentially by the end of the training as he is able to shatter large boulders without much effort and even fend himself against Saibamen which were strong enough to keep all the Z Fighters, such as Tien and Yamcha, on their toes.
8 Kid Gohan (Frieza Saga)
In order to wish back their dead friends who had already been revived with the Earth Dragon Balls, Gohan, Krillin, and Bulma travel to the distant planet Namek in order to use their Dragon Balls. During their quest, Gohan encounters Guru, the elder Namekian who served as a leader among the rest, who after sensing the danger Frieza and his army, decides to help them and unlocked some of Gohan's hidden potential.
This power boost allowed Gohan to face off against Frieza in his first form and even gain a split second advantage against him, leaving Vegeta stunned and curious of his potential.
7 Adult Gohan
After defeating Cell and returning peace to the planet, Gohan and the gang live their days without worries. By the time we are introduced to the Buu Saga Gohan has grown into a young adult and is just starting high school. Focusing on his studies and taking a long break from fighting led his power to deteriorate.
He only had an echo of the power he had in his teenage youth when he faced off against Cell. Nonetheless, he is still far stronger than the other Z Fighters apart from Goku, who was dead, and Vegeta who constantly trained since the battle with Cell and reached a level of strength higher than him.
6 Teen Gohan
After Future Trunks warns them about an approaching danger, the Z Fighters prepare themselves for the worst and start training to get stronger. When the Androids arrive, the prediction proved to be inaccurate. There were two extra Androids, and there was a mysterious biological android known as Cell.
After gaining his full power, Cell threatened the world and announced a tournament in ten days. Gohan spent a single day in the Hyperbolic Time Chamber with Goku and finally attained the Super Saiyan transformation. This made him stronger than even Vegeta in his base form, allowing Gohan to handle Androids with ease.
5 SSJ Adult Gohan
Due to a lack of training and being more focused on his life as a normal citizen, Gohan began to lose his strength and eventually reached a point where his Super Saiyan transformation, albeit impressive, gave him only a fraction of the power boost that it did in his prime fighting days during the Cell saga.
The transformation is only used by him as a form of disguise to stop a few criminals and is known to trigger very easily, especially if he is angered. Unfortunately, because Gohan was no longer in peak condition, instead of being a game-changing maneuver, it was simply a tool to make himself look stronger than he actually was.
4 SSJ Teen Gohan
After entering the hyperbolic time chamber with his father, Gohan underwent strict training in order to achieve more power and become a more formidable opponent for Goku to spar against.
During the training, Gohan is taught by Goku on how to channel his anger and attain the Super Saiyan transformation without the need to train excessively as Vegeta had or experience an emotional trigger as Goku himself did. Gohan was able to learn the transformation and perform it on command, making him the youngest Saiyan to achieve the legendary form at the time. He is strong enough to survive a one on one against Cell even though he was still greatly overmatched.
3 SSJ2 Adult Gohan
The transformation that was the next level of Super Saiyan makes its return in the Buu Saga during Gohan's battle with Kabito. Gohan achieved this transformation after Cell destroys Android 16 in front of him which causes him to snap. The form uses his energy to reinforce his muscles which increases the muscles mass and strength without sacrificing speed and causes electric sparks to surround his body.
These sparks, according to the lore, are caused by stored kinetic energy. Though it still retains its properties, the form has been greatly weakened because of the break from fighting. Vegeta states that Gohan has gone soft over the years.
2 SSJ2 Teen Gohan
First appearing during the battle with Cell, Gohan was able to become the strongest fighter in the universe by even surpassing Cell and his father in terms of power and speed. After obtaining the transformation, Gohan is able to make quick work of all the Cell Juniors in one blow each.
A simple kick in Cell's stomach caused him to throw up the absorbed Android 18, which, in turn, forced him to revert into his Imperfect Form. His regular Kamehameha was able to overpower Cell's full power Solar Kamehameha and his full power Kamehameha was strong enough to disintegrate Cell. This is the strongest form a child in the series was able to attain, with the exception of the fusion, Gotenks.
1 Mystic Gohan
This is the strongest form Gohan has been able to achieve up to date. After being defeated by Majin Buu and being on the brink of death, he was brought back to the world of the Kais and trained using the Z Sword. After breaking the sword in two and releasing the Old Kia, Gohan undergoes a 24-hour ritual to unlock all his potential.
After the ritual is completed, Gohan obtains power which is stronger than his Super Saiyan 2 and it doesn’t even change his appearance. He easily overpowers Super Buu and toys with him, to the point of error because his attitude caused both Piccolo and Gotenks are absorbed. Despite this, Gohan holds his own.