Gegege no Kitaro is a manga first published in 1960. Kitaro is the last of the ghost tribe, a link between humans and Yokai. He is not one nor the other but stands on the line between. The manga and the anime (which the latest was the sixth iteration aired in 2018) hardy allowed more than one chapter for his backstory (manga) and the anime decided it was so iconic that it didn't even need one, it was just rehashed in one of the OPs visuals. The movie starts before this as Mizuki, a reporter, is sent to the countryside to investigate a story. Thisturns into a murder mystery ignited via a dispute over inheritance with supernatural elements hanging over the tale.

Story is great, animation is amazing without the use of CGI.
It has some dark elements which include Incest and body horror so something to be aware of before going in.

Do you need to know anything about gegege no Kitaro to watch this?

Honestly, no. The only link is that the ending is the iconic birth of Kitaro that is the starting point of the manga but that's about it.

I do think fans of horror, hand drawn animation & yokai will like this. Fans of the show will love it though the usual cast barely make a cameo as this isn't their story so don't go in expecting to see them much.

Overall a very atmospheric movie.