ISFx would actually be a good guess. I don't know as much about Gein as I do about Bundy, but I do know he would watch neighborhood children, everyone thought he was nice, but as a child he was horribly picked on. Kind of a gentle but dangerous person. Like Norman Bates, who is a composite of him, where as a character like Patrick Bateman would be more like Ted Bundy.
An ISFP possibly. He was quite the ...er...artist.