How does gohan look in his ultamate form?

Answer 1

He wears the clothes of Goku and his hair looks like a SSJ2 but black. His power surpasses a SSJ3. Here's some info from Dragon Ball Wiki.

Gohan's ultimate power up when the full extent of his dormant power is released during the Fusion Saga. This power up was achieved by a special ceremony performed by Old Kai. Although technically this ability is not exclusive to Gohan alone as the ceremony can be used to unlock anyone's sleeping powers, his was unusual in the amount of time it took to unlock due to his amazing potential unlike any other person who had undergone this ritual before. Gohan activates this ability in a similar fashion to the Super Saiyan transformation, however the colors of his aura, hair, and eyes do not change. Only subtle differences are present, including more pronounced facial features, more upright hair, and a large, uncolored aura. This power up is referred to as Strongest Warrior in Dragon Ball Z: Legendary Super Warriors, Elder Kai's Unlock Ability in the Budokai series and Ultimate Gohan in the Budokai Tenkaichi series. When first attained, Gohan's strength easily surpasses all of his other forms. Gohan's power in this form is on par with that of Super Buu after absorbing Goten, Trunks, and Piccolo, though Super Buu eventually outmatches Gohan due to his unique techniques and his absorbed victims' skills.

In the movie Wrath of the Dragon, Gohan is shown "powering up" into his ultimate state on several occasions during the battle against Hirudegarn, neglecting his multiple available Super Saiyan transformations. This movie is the last time Gohan used this transformation. In the same movie, Super Saiyan 3 Goku is shown to be more powerful than Ultimate Gohan; Gohan cannot hit Hirudegarn or block any of his attacks, while Super Saiyan 3 Goku almost single-handedly defeats the monster. This is partially due to Goku's superior fighting intellect, which led to his discovery of Hirudegarn's weakness (becoming angry whenever it is insulted).