The most common way to refer to a grandmother's sister is a great aunt. Some refer to this relation as a grandaunt, and is the aunt to the grandmother's children.
A grandmother's sister's children are second cousins to the grandchildren of the grandmother and first cousins once removed to the children of the grandmother. A grandmother's brother is most commonly referred to as a great uncle or a granduncle by the grandmother's grandchildren.
In contrast, a great aunt commonly refers to the grandchildren of her sister or brother as great niece or grandniece for girls and great nephew or grandnephew for boys.