姑妈 refers specifically to the married sister of one's father (paternal aunt by marriage). This is distinct from 姨妈 (maternal aunt, mother's sister) and 姑姑 (father's sister, which can be used for both married and unmarried, but 姑妈 explicitly indicates she is married). Use this term when talking about your father's married sister, her husband (姑父), and their family.