I'm sorry Ermintrude, but I'm with everyone else on this. It is their baby, and they can call it whatever they want to. Unfortunately the people who are losing here are you and the baby; you are missing seeing a beautiful child growing up, and the baby is missing the love and fun a child gets from a grandmother.
My daughter has two children, both of whose names are unusual, but we realised very early on that they are their children, not ours, Maybe our daughter didn't think much of the names we chose for our children (her and her brother), but we chose them because they were our children to name, and she and her husband chose the names of their children for the same reason.
Please don't deny yourself and the baby the chance of a wonderful grandmother/grandchild relationship just because you don't like the baby's name.