How to Make It
Combine 2 cups fresh blueberries, 2 cups fresh blackberries, 1 1/2 cups granulated sugar, and 1/2 cup water in a large saucepan. Bring to a boil over medium-high, stirring until sugar is dissolved. Reduce heat to low, and simmer until berries have softened and begin to break down, 5 to 7 minutes. Remove from heat, and cool completely (about 1 hour).
Pour fruit mixture through a fine wire-mesh strainer into a pitcher, discarding solids. (Yield should equal about 2 1/2 cups.) Stir in 1/2 cup fresh lime juice. Cover and chill until very cold, about 4 hours.
For each serving, pour about 6 tablespoons of the syrup over ice in a 12-ounce glass and stir in 3 tablespoons gin. Top each with about 2/3 cup club soda. Garnish with blueberries, blackberries, and lime wedges.