In a small saucepan, sprinkle the gelatin over the water and let it soften for 5 minutes.
Add the sugar, and stir over very low heat until the gelatin and sugar are dissolved.
Puree the strawberries in a food processor.
Puree the watermelon.
Combine the two and strain the mixture through a mesh strainer.
Discard the remaining solids (which should be no more than 1/2 cup) and pour the fruit into the container of an ice-cream maker.
Stir in the gelatin mixture.
Freeze the sorbet according to the manufacturer's instructions.