In small bowl, dissolve gelatin and sugar in boiling water; add mint leaves. Cover and steep for 20 minutes. Press through a sieve; discard mint. Stir in the limeade concentrate, cold water, soda and rum. Pour into a 2 1/2qt freezer-proof container. Freeze overnight.
For each serving, scoop 1 cup slush into glass. Pour 2/3 - 1 cup grapefruit soda into glass. Garnish with mint leaf if desired.