Instructions
Leave the area where Babidi's ship is located and go back to any of the previous areas. If you've been fighting your way through this chapter with anybody other than Gohan, switch your current character for him. The "85" on the door is color-coded green, which is Gohan's color.
Fight as many enemies as you can. If you deplete an area's enemies, move to another area and then return for them to respawn. You can also keep fighting the Destroyer, who guards the path to Babidi's ship, to get more experience.
Keep fighting enemies until Gohan is at least level 85. Return to Babidi's ship and punch the ship's door to have it open.