My maternal grandparents were Nana & Grandpa. My paternal grandparents were Grammy & Poppy Chew (their last name).
My boys call my mother Grandma. My paternal grandparents were dead by the time my kids came along, but they knew my maternal grandparents and called them Nana & Grandpa as well (there were no "real" grandpas around at the time, so there was no confusion). All of my grandparents are gone, now. In fact, Saturday it was 10 years since we buried my Nana...still hurts like it was yesterday...
Anyway, the boys alternatively call my in-laws by their first names or Gran & Grandad.
They don't call my father anything. They met him once (at my wedding!) and he's never been a part of their lives. They also have no relationship with the parents of their "fathers," nor with their "fathers," either.