Both twins should be getting close to being confirmed and Gabriella can wear white for Confirmation even if she would be a teenager. It's just when the family should visit the Vatican and the Pope or a Papal visit to Monaco that Gabriella would have to wear black once she reaches her teens.
Princess Alexandra of Luxembourg wore white for Papal visits as a little girl and then black when she was older. A few years ago the Lux. Grand Ducal family met with Pope Francis and Alexandra wore black. Her mother was in the Privilege du Blanc.