Use a cheese slicer or vegetable peeler to create a long thin strip of cucumber. You may have to peel a few slices to get to the middle of the cucumber.
Place the cucumber slice inside of your serving glass, and push it against the side of the glass so it sticks.
Cut a slice of lime to add to the rim of the glass at the end. Cut a small notch in the peel, and set it aside for now.
Cocktail
Add your cucumber slices to your cocktail shaker. Use a muddler to break up the cucumber so that the juices come out.
Add your lime juice to the cocktail shaker. Then, add your simple syrup, lime bitters and gin.
Fill the cocktail shaker with ice, seal it up and give it about 15-20 good shakes.
Fill the serving glass with fresh ice, and strain the drink into the glass. If possible, use two strainers (one regular and one fine-mesh) together.
Top the drink with seltzer.
Add the lime slice to the rim of the glass for garnish, and serve.