Well I never had children in the 27 years of our marriage so Grandbabies are out of the question for me.
Consider yourself blessed.
You do bring up an interesting topic and I started thinking what I would like to be called if I was in that situation.
I remember calling my fathers grandma Busia, a Polish term, and Grannie for my Moms mother, she didn't like that.
I think we were influenced by the TV show Beverly Hillbillies.
I really like the nickname "Tootsie" if I had grandkids who interacted with me while I presented as a female lady.
Another cute one would be "Birdie."
Here is a link to a article on the subject.