The German version of Grandma is Oma or Omi. Both are possible. Oma is also sometimes used in a derogorative way, particularly when talking about people who are not your grandparents. I never heard the word Omi to be used that way. In my opinion it's even more of a term of endearment than Oma. Great-Grandma would be Uroma.
The male versions are Opa and Uropa.
Perhaps you could use something along the lines of GranGranny and GranGramps, if that doesn't add up to something entirely impossible for native speaker's ears.
