Pour gin, Campari, vermouth, grapefruit juice and Simple Syrup into an airtight container. Freeze at least 6 hours and up to 3 days.
Place 2 Lowball glasses in freezer for at least 30 minutes.
Pour drink mixture into blender and add ice. Blend until smooth. Divide among chilled glasses with a spoon. Garnish with orange slices and serve with straws.
SIMPLE SYRUP
Combine equal parts sugar and boiling water, stir to dissolve, and store in fridge until chilled.