If your great grandmother was his great aunt you and he are second cousins, once removed.
Your great grandmother's sister is your great-great-aunt.
Your grandmother's sister is your great aunt
That is correct, like your Dad's grandmother is your great grandmother.
A great great aunt is the sister of your great grandmother. (A great aunt would be the sister of your grandmother.)
Your great grandmother's sister is your great great aunt (or some would say, your great grand aunt).
Yes, your great grandmother's sister is your great-great aunt. Your great aunt is your grandmother's sister.
If the great grandmother of A is the aunt of B's grandfather, then A and B are third cousins.
If your grandmother was his great aunt then you share a great grandparent and are second cousins.
Your great aunt's mother is your great grandmother.
Your grandmother's sister is your great aunt. Your grandmother's sister's daughter is your first cousin, once removed. (Your grandmother's sister's daughter and your mother or father are first cousins, and going one more generation to you on only one side is the "once removed" part.)
Your great grandmother's sister is your great great aunt and your son's great great great-aunt. He is her great great great nephew.
Your grandmother's sister is your great aunt. Your great aunt's daughter is your first cousin, once removed.