Is my father-in-law's mother (my wife's grandfather's mother) a mahram to me?

Your wife's maternal grandmother is permanently forbidden to you in marriage, because whoever marries a woman, her ascendants — such as the mother, the mother's mother, and grandmothers, however high they may be — become forbidden to him. This is based on the Almighty's saying: "and your wives' mothers"; the term "mother" here refers to every female from whom a birth relationship stems for you.