Praise be to Allah.
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).