I called them Grandma/Grandpa (First Name) My kids same thing with Grandmas but one wanted to be Nana not Grandma...they only had one living Grandpa who they call Papaw. My grandkids say Nana and Papaw.
I only knew my maternal Grandmother and only met her a few times. I called her grandma. I am Grandpa to my blood grandkids. My step-grandson calls me Papa.
Actually I never had much chance, and my siblings had none, to call one Grandad anything except in reference, because he died so early I hardly knew him.
Our expanding brood continue the tradition but as far as I am aware I am not known as, nor have I been called, "Great Uncle", just "Uncle" [Durdle].
The first time my sister told me her daughter had had a baby, she said, "So now you are "Great Uncle [...]!"
"Oh thanks," I replied. "Makes me sound like an elderly Womble!"