We call my sister
Geneva &
Gigi interchangeably. The nn seems so normal to me now that it's hard to step outside and think about it objectively. Her middle name is
Grace, so G.G. are her initials.
I don't think she would like it if we called her
Gigi exclusively. "
Geneva" is more mature and she introduced herself as such. But it is fun to have a special pet name among family & close friends.
I could see
Gigi being used for a long, hard G name, but something like
Greta (which is already 2 syllables and a nickname for something else) would make even less sense.