Cut the mango off the stone. Peel the fruit flesh and cut into large pieces. Put mango, lime juice, sugar, ginger beer, gin and ice cubes into a blender and mix until a creamy, sorbet-like consistency is obtained.
Wash the rosemary and dab dry. Wash the mint, shake dry and pluck the leaves from the stalks. Pour the mango mixture into glasses and spread rosemary and mint in them.