Praise be to Allaah.
Your grandfather’s daughter must necessarily be your
maternal aunt (khaalah) or paternal aunt (‘ammah). If she is the daughter of your
father’s father, she is your paternal aunt, and if she is the daughter of your
mother’s father, then she is your maternal aunt. If she is your aunt, whether the
sister of your father or the sister of your mother, then you are a mahram for her,
regardless of whether she is married to your uncle or not. It seems from the question that
she is the daughter of your maternal grandfather (your mother’s father).