There are manga, light novel, and anime versions of Code Geass. Which came first?
It was original, the anime came first then various adaptations. You could confirm this in MyAnimeList. Under "Background":
Code Geass: Hangyaku no Lelouch is an original anime series by Sunrise animation studio with original character designs by the all-female Japanese manga artist group Clamp.
Just don't be surprised that some good shows are anime-original like Psycho Pass, Madoka, etc.