What Would I Call My Grandfather's Brother?
Your grandfather's brother is your great-uncle. In some countries, he may also be called a granduncle.
Similarly, your grandfather's sister is your great-aunt or grandaunt, and you are her great-nephew (grandnephew) or great-niece (grandniece). A great-aunt or great-uncle is also your mother's or father's aunt or uncle. Great-uncles, great-aunts, great-nephews and great-nieces are part of the immediate extended family in most genealogy structures.
Your spouse's grandfather's siblings are your great-uncles-in-law or great-aunts-in-law, and you are their great-nephew-in-law or great-niece-in-law.
Of course, kinship and familial relations are often different in cultures outside the United States. Thus, it is always important to study what relationships are deemed close family and extended family for your particular heritage.