Step 1

In a small pot over medium heat, combine the sugar and water. Bring to a boil, stirring to dissolve the sugar completely.

Step 2

Remove the pot from the heat and stir in the cucumber. Let steep, covered, for at least 4 hours or up to overnight.

Step 3

Strain the syrup through a fine-mesh sieve or cheesecloth, pushing to extract as much syrup as possible. Store in an airtight container in the refrigerator for up to 1 month.

Step 4

In a cocktail shaker filled with ice, combine the gin, watermelon juice, lime juice, and simple syrup. Shake vigorously for 20 seconds.

Step 5

Strain into a glass filled with ice and top with club soda.

Step 6

Garnish with the cucumber slice.

Step 7

Enjoy!