Put the eucalyptus leaves in a saucepan with 600 ml (21 fl oz) water, cover and place over low heat. Leave to steep for 3 hours. Stir in the sugar and chill in the fridge. Add the gellan gum and heat to 84°C (183°F), stirring constantly. Pour into a container and then chill and leave to set in the fridge. Remove the leaves, purée until smooth in a blender and then keep chilled.