Soak the gelatine in cold water for 5 minutes or until softened.
Place the rum, grapefruit juice, lime juice, maraschino liqueur and sugar syrup in a bowl and stir to combine. Transfer 100 ml (3½ fl oz) to a saucepan over medium heat. Squeeze out the excess water from the gelatine and add the gelatine to the pan, stirring until dissolved. Allow to cool, then combine with the remaining rum mixture.
Pour into a cream canister and charge twice. Discharge 150 ml (5 fl oz) of the rum mixture into a stainless steel bowl. Add 100 ml (3½ fl oz) liquid nitrogen and, using a spatula, stir and break into popcorn-sized pieces. Place in a martini glass, scatter over the lime zest and serve immediatley with a teaspoon. Repeat with the remaining rum mixture.