Chill a highball glass. Fill a cocktail shaker with 6 of the ice cubes and add the gin, lime juice, lemon juice, Cointreau, and sugar syrup. Shake vigorously to blend and chill.
Fill the chilled glass with the remaining ice cubes. Strain the gin and juice mixture into the glass, leaving out what's left of the original 6 ice cubes. Add enough seltezer to top the glass off. Garnish at your pleasure.
Bring water to a boil in small saucepan. Remove pan from heat and add the sugar. Stir until the sugar is completely dissolved.
Let cool completely before using or refrigerating. Pour unused syrup into a clean glass jar and cap tightly. Store in the refrigerator and use as needed.