One's grandmother's brother is usually called a great-uncle, in terms of familial relationships. Grand-uncle is another name for this kin, but the term isn't used as often.
One's grandfather's brother is also termed great- or grand-uncle. Similarly, the sister of a grandmother or grandfather is a great- or grand-aunt. Reversing the relationship, the grandchild is technically a great-niece or great-nephew to this extended family member, depending on the gender. Labeling relatives gets trickier when step and half relations are involved and when dealing with generations that are "removed," in the jargon of genealogy. Prefacing these labels with paternal or maternal indicates whether the relative is on the mother's or father's side of the family.