Fill a highball glass with ice cubes. Give it a quick stir to chill well and set it aside.
Measure and pour ¾ oz simple syrup into the shaker.
Add ½ oz fresh lemon juice.
Add 1½ oz fresh grapefruit juice as well.
Finally, add 2 oz Gin to the mix.
Fill the shaker with ice, close it and give it a good shake (10-15 seconds).
Take the Collins glass and use the cocktail strainer to remove any excess water inside or replace the ice.
Using the cocktail strainer, strain the mixture from the shaker into the ice filled glass.
Top up with 3-4 oz club soda and stir gently to combine.
Finally, place a grapefruit slice or twist into the glass for garnish. Serve immediately.