I saw the English version at an art cinema in 1987. Rene's film Fantastic Planet is visually interesting, but not a story for American tastes. Gandahar, aka "Light Years" is even further away from any kind of storytelling Americans would like.
The film is more of a philosophical question of what kind of life is more deserving of survival...common folk, mutants, or super-intelligence?
People are being killed, and a "hero" sent to protect them forgoes taking direct action when the source of trouble - a super-brain, claims innocence and needs time to sort things out - 1000 years.
At this point, the movie is unwatchable, because the "hero" decides to wait it out and lets people suffer for 1000 years before taking any action.
Unfortunately, while suffering this indignity of a storyline, you have to watch rather plain, unattractive design and animation.
Avoid for a 1000 years.