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1
Bring sugar, water, and zest to a boil in a 2-quart heavy saucepan, stirring until sugar is dissolved, then reduce heat and simmer, without stirring, washing down any sugar crystals on side of pan with a pastry brush dipped in cold water, 2 minutes.
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2
Put watermelon in a blender, then add syrup and lemon juice and blend until smooth.
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3
Pour through a fine-mesh sieve into a large bowl, pressing on pulp and discarding any remaining solids.
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4
Freeze in ice cream maker, then transfer to an airtight container and freeze until firm.
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5
Blanch basil in a 3-quart pot of boiling salted water 5 seconds, then drain and transfer to a bowl of ice and cold water to stop cooking.
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6
Drain basil and squeeze dry.
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7
Bring wine, sugar, zest, and 1/2 cup water to a boil in cleaned 2-quart heavy saucepan, stirring until sugar is dissolved, then simmer 2 minutes.
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8
While wine syrup simmers, sprinkle gelatin over remaining 2 tablespoons water in a large metal bowl and let soften 1 minute.
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9
Pour about 1/2 cup wine syrup through a medium-mesh sieve lined with a dampened paper towel into gelatin mixture (reserve lined sieve), then stir until gelatin is dissolved.
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10
Blend remaining syrup with basil and lemon juice in cleaned blender until smooth (use caution when blending hot liquids), then pour through lined sieve into gelatin mixture.
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11
Set bowl in a larger bowl of ice and cold water and let stand, stirring occasionally, until cold, about 15 minutes, then chill in refrigerator, uncovered, until set, at least 2 hours.
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12
Gently whisk gelee to break into small pieces and serve as a bed for scoops of sorbet.
