I love to hear nicknames people have for their grandparents. More often it seems it's the grandFATHERS who get the nicknames. Grandmother may get something like "Nana" but the really odd ones that come from oldest grandchild tend to go to grandpa's. Names like "Far-far" and "Hi-Guy."
My grandpa was "Bente." "Bente" was named by my sister. I believe it came from my grandpa trying to call her over "Vente! Vente!" (Come here.)
So for us and my cousins it was always Abuela and Bente. In fact because it was so close to the spanish number "veinte" sometimes cards we got from them would be signed "Abuela and 20."