Normally the first grandchild will decide on what you will be called; you don't really have much of a say, unless you are very persistant.
However, what about your board name, 'Mamagus'? I really like that...
My name is Kathy, and I was not yet married to my husband when our first granddaughter was born, and since all his adult children called me Kathy, & the first granddaughter had difficulty saying that when she started talking, she called me 'Kaky', which is what all the rest of the grandchildren have called me, and I love it. It is definitely unique...