Your grandpa's mother would be your great-grandmother. You add another great for each generation you go up. Your grandpa's sister would be your great aunt.

That depends whether you are talking to them or not. Your grandmother's mother is you great grandmother, but you should call her "grandma". Your grandmother's sister is your great aunt, or if you are writing a book, write "my grandmother's sister". If talking to her say: Auntie [name].