My parents are Grandma and Grandpa. When I was a kid we called our grandparents Grandma and Grandpa 'last name'.
DH's parents were Grammie and Papa. Grammie was traditional for their family, and Papa came because my oldest nephew turned Grandpa into Papa. My DDs were the youngest grandchildren, and DD#1 turned Grammie into Googie.
Grammie did not like it at all at first because I think she thought it was something we taught her. But when she realized that DD did it herself, and did it to lots of things that she loved (her bunny was her ****ie, horses were hoobies), she decided it wasn't so bad. My DDs were the only grandchildren who called her that.
When Googie died, the ribbon on her flower arrangement from the grandchildren, chosen by my sisters in law, said "loving Grammie" on one side, and "Loving Googie" on the other.