Both of my grandmothers were simply Grandma. We called both of them that to their face. And as others have said, if we needed to differentiate between them (talking among my siblings), we added the last name. I also had two Grandpas. (Was fortunate to have all four into adulthood.) So I don’t feel like they HAVE to have distinct names; we had no issues at all, wasn’t a big deal.
DH called the only grandmother he knew Nana, then Nan when he got older (same as when kids transition from Mommy and Daddy to Mom and Dad). So with DS, DH’s mom is Nana/Nan, and my parents were Grandma and Grandpa.
I’m okay with some of the traditional or ethnic variations: Granny, Nanny, Memaw, Oma, etc. However, with no disrespect meant, I honestly don’t understand why people need to make up something completely different or let the child choose what to call them. Because most wouldn’t/don’t do that with parent names. If you refer to yourselves as Mommy and Daddy, when the child starts speaking, then that’s what they’ll call you. And if there are speech/pronunciation/baby talk issues, they usually clear by pre-school age. Sorry, I just don’t get it. I’ve heard some women use the excuse that they’re “too young to be a Grandma.” Please. Own it. In the greeting card section in a store (and I realize cards are falling out of favor), they all say (with rare exceptions), Grandma, Grandmother, or Nana. No Gigi or Bobo.
Should I be blessed to have grandchildren some day, my preference is Grandma, a name I would happily and proudly cherish.