In my own family my grandmother is Nana (still living), and my great-grandmother was grandma, mostly because my mom called her grandma, and she was gone while I was still very young, so there was no reason to change it.
From there, back, my great-grandmother's mother is just called Grandma Rose. It takes too much time to throw in as many greats as you'd need, especially when talking to family members with different levels of "great" removal. When we say "Grandma Rose" we all know about whom we speak.
My boyfriend, on the other hand, uses different terms for his maternal and paternal grandmothers, though that doesn't cross up into the generational aspect. One is "Oma", and I think the other is "Nonna". (He gets grouchy when he accidentally refers to her as his "Nana" for how often he interacts with my grandma these days.)