While fans assume that Dominique is a girl, and that does seem likely, there is nothing actually stated in canon to verify gender. The name is unisex and could belong to either a boy or a girl (Dominique on babynames.com). A notable example of a male with that name is Dominique Wilkins, a professional basketball player. Even Pottermore’s artists make this assumption, as shown in the Weasley Family Tree they posted. The only time Rowling herself has ever mentioned Dominique is in the television documentary “J.K. Rowling: A Year in The Life” and all she gave was the name.