Yes, there's a sort of prequel called
Saiyuki Gaiden, that is about what happened 500 years before the facts of
Gensomaden Saiyuki. It's basically on the origins (and previous existences) of the main characters in Gensomaden Saiyuki and explains many aspects that are not so clear simply watching or reading the sequel.
If you want some more info, you can look at this
site. There aren't many on the web, mainly because this series is still unknown for the western audience.