For my childhood: Either set could be “Grandma” and “Grandpa”. One grandfather could also be “Grandpap” or “Pappy”, and the other could be “Pops” or “Grandpop”, but neither was consistent. If disambiguation was needed, we used last names. I barely knew one of my great-grandmothers, and she was also “Grandma Lastname” (fortunately Grandma’s mother, not Grandpap’s, so the last name was different)
For the next generation, my sister’s children: Their father’s side is just “Grandma” and “Grandpa”, but my mother is “Gigga” (which originated from baby-talk), and my father wasn’t in the picture for them. While my grandmother (their great-grandmother) was still alive, she was “Mazzie”, which was apparently one of Grandpap’s nicknames for her.