I never met any grandfathers but they were always referred to as "Grandpop Lastname"
My maternal grandmother (only grandparent I knew) was Grandmom.
The ones that were born before she died called her Great Grandmom.
My parents were Mom Mom and Pop Pop to all their grandkids. (They were 38 and 39 when my nephew was born and my mom didn't want to be "Grandmom" or anything like that).
My In Laws are (were) Granny and Papa. My FIL has since passed away. My BIL is Papa to his grandson. Almost everyone calls my MIL Granny now even non-grandkids, LOL
Most of us have shortened my dad's Pop Pop to just Pop in recent years, the grandkids and great grandkids call him that.