A "great uncle" refers to any brother of either grandparent. This is also the same as being the uncle of one's father or mother.
Consider Tom, who has two parents, Bill and Jessica. As Tom's father, Bill has parents Mike and Rose (Tom's grandparents), whom combined, have siblings Manny, Shawn, Peter, Sally, Sharon and Gwen. Tom's mother, Jessica, has parents Steve and Carol (also Tom's grandparents), who, together, have the siblings John, Joseph, Larry, Tammy and Norma. Manny, Shawn, Peter, John, Joseph and Larry would all be considered great uncles to Tom, while Sally, Gwen, Norma, Sharon and Tammy are Tom's great aunts.