"Tanta" means "aunt" in Arabic. The term is often used to refer to the sister of one's parent. In contrast, the word for grandmother in Arabic is "jaddah."
That is correct, like your Dad's grandmother is your great grandmother.
Your grandmother's daughter is your mother or your aunt.
Your great grandmother's sister is your great-great-aunt.
Your grandmother's sister is your great aunt
If the great grandmother of A is the aunt of B's grandfather, then A and B are third cousins.
If your great grandmother was his great aunt you and he are second cousins, once removed.
A great great aunt is the sister of your great grandmother. (A great aunt would be the sister of your grandmother.)
Your grandmother's sister is your great aunt.
Your grandmother's sister is your great aunt.
If your grandmother was his great aunt then you share a great grandparent and are second cousins.
Your mother's mother is your grandmother. Your grandmother's sister is your great aunt, and your mother's aunt.
Your great grandmother's sister is your great great aunt (or some would say, your great grand aunt).