Is it permissible to marry any of my grandfather's cousin's daughters? I am instructed to call them "aunt". I understand it is not permissible to marry your mother's sister or father's sister, but do these rulings apply in this circumstance?
"And do not marry those [women] whom your fathers married, except what has already occurred. Indeed, it was an immorality and hateful [to Allah] and was evil as a way. Prohibited to you [for marriage] are your mothers, your daughters, your sisters, your father’s sisters, your mother’s sisters, your brother’s daughters, your sister’s daughters, your [milk] mothers who nursed you, your sisters through nursing, your wives’ mothers, and your step daughters under your guardianship [born] of your wives unto whom you have gone in. But if you have not gone in unto them, there is no sin upon you. And [also prohibited are] the wives of your sons who are from your [own] loins, and that you take [in marriage] two sisters simultaneously, except for what has already occurred. Indeed, Allah is ever Forgiving and Merciful. And [also prohibited to you are all] married women except those your right hands possess [i.e., slaves or war-captives who had polytheistic husbands]. [This is] the decree of Allah upon you. And lawful to you are [all others] beyond these, [provided] that you seek them [in marriage] with [gifts from] your property, desiring chastity, not unlawful sexual intercourse..." [Quran 4:22-24]
Now coming to your question can you marry your grandfather's cousin's daughters? The answer is yes you can because your grandfather's cousin's daughters are not in the list of who is unpermissible to marry.
YES you can marry her.
Because your grandfather's cousin's daughter is not your mahram. She is not even mahram to your grandfather too.