DB Chapter 208
From Dragon Ball Encyclopedia, the ''Dragon Ball'' wiki
"Deeds Done by the Full Moon" (満月の出来ごと, Mangetsu no Dekigoto; Literally meaning "An Incident Under the Full Moon") is chapter 208 of the Dragon Ball manga.
There's a full moon out, and Gohan is sleeping atop a mountain. Piccolo is also sleeping, while sitting cross legged, and floating mid-air. Gohan then wakes up to take a piss, and worries about how he'll eventually get down from here. Then he wonders why it's so bright out at night, and turns around and looks at the moon. This is his first time seeing a full one. And then he gets the look in his eyes like Goku used to, and begins transforming... Ōzaru Gohan is too big to stand atop the mountain, and so he falls to the ground.
Piccolo freaks out as Gohan starts smash everything around him and fires a huge mouth blast off into the distance. He thinks the Earth will be destroyed before the Saiyans even get here, and then remembers what Raditz said to Goku about full moon transformations, and so he decides to take out the moon. Gohan then returns to normal, and falls asleep naked on the ground. Piccolo starts thinking about the strength of these Saiyans, and then realizes the tails are also needed for transformation.
So, Piccolo pulls the tail off, and then decides to make Gohan a sword, and an outfit just like his father's. Well, with the ma ("Demon") kanji instead of kame ("Turtle"). In six months he'll get special Hell training from Piccolo. He's going to make him into one of the Demon Tribe. Then he flies off and leaves Gohan to sleep peacefully, while Goku is also sleeping peacefully on Serpent Road.