He’s a blundering evil black dragon type who was sick of performing menial tasks in Heaven. He foolishly challenged ZHU-RONG, the Heavenly God of Fire, to a fight. They battled furiously for days and then fell out of Heaven, still throwing punches.
Upon hitting the Earth, GONG-GONG was much the worse for wear and ZHU-RONG, claiming victory, ascended to Heaven for a lap of honor.
GONG-GONG was so humiliated at this defeat that he decided to kill himself by head-butting one of the pillars of Heaven. This had the desired effect, but also ripped a great hole in the firmament, causing the Great Flood which devastated Earth.
It was up to NU-GUA to put things right, but even she couldn’t undo all the damage. The very Heavens were tipped sideways, and that’s why the Pole Star is no longer in the center of the night sky.