What is the ruling on marrying a girl who was suckled by my maternal grandmother (my mother's mother) for a full year, without specifying the number of sucklings, and whether she was satiated?

If the girl was nursed by your maternal grandmother five known sucklings within the first two years of her birth, then she is considered your grandmother's daughter through suckling. Consequently, it is not permissible for you to marry her, because she is considered your maternal aunt through suckling.