Bring the sugar and lime juice to a boil in a small saucepan, stirring until the sugar has dissolved. Let cool, then transfer the syrup to a 3-quart pitcher.
Add the wine, gin, triple sec and soda to the pitcher, and stir to combine. Add the cucumbers, jicama and melon, and cover the pitcher with plastic wrap. Refrigerate the sangria for at least 4 hours up to overnight.
To serve, fill 8 glasses halfway with ice, then pour the sangria and fruit into each glass.
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