It's nothing specially complicated. I used my most simple jersey bodice pattern and my standard sleeve pattern that I just lengthened. The skirt is simply a full width of fabric gathered to the waist. It's not the most flattering, but it's just an undergown. Since there is quite a lot of fabric I stabilized the neckline by binding it with bias tape. As a bonus I know have a gown to wear if I ever want to dress up as a gingerbread woman.
The problem with having spent so much time looking at the reference photos is that of course you realize that a lot of the assumptions you had made were wrong. I have ordered new fabric for the outer tunic, since I just couldn't stand my first choice. I wanted something that was a lot more drapey.
The good thing with ordering new fabric is that I have some time to figure out the outer tunic, it's a lot more complicated than I first thought.