1. In a medium saucepan, combine the water, sugar, rosemary, and lemon zest. Bring to a simmer, stirring occasionally. Remove from the heat, cover, and let infuse for 10 minutes.
2. In a large glass or stainless-steel bowl, combine the lemon juice and Grand Marnier. Strain the rosemary syrup into the lemon juice and stir to combine. Chill in the refrigerator or, to hasten the process, pour the lemon mixture into a stainless-steel bowl, set it in a larger bowl filled with ice, and stir until cold.
3. Pour the chilled lemon mixture into two 9-by-9-inch stainless-steel pans. Freeze for 15 minutes. Stir well and return the pans to the freezer. Continue freezing, stirring every 15 minutes, until the granita is completely frozen, about 1 hour in all. When ready to serve, scoop the granita into chilled bowls or stemmed glasses.