This role is usually by a small girl child, a friend’s child or niece or maybe even little sister. Or sometimes even the family dog. No matter, the child should be outfitted according to the theme of the wedding, which can often means something frilly, in Victorian, Medieval or Renaissance with ruffles and some sort of little headdress, like a crown or bow.
The flower girl set doesn't need to consume much thought. It’s pretty simple and the little girl is usually very happy to play dress up and wear whatever you choose. Whatever ideas you select, just keep them in tone with your "romantic wedding dresses" also overall wedding theme and use the flower girl dresses to build further on your gothic theme.