Salt the rim of your glass, use your garnish to rub on the rim so the salt sticks.
In a shaker with ice, add your gin, grapefruit juice, and simple syrup.
Shake it up.
Add ice to your salted rocks glass, and pour in your gin and juice mixture.
Top with soda or seltzer to finish.
Garnish with a wedge of something - grapefruit if you have it, lemon or lime is just as tasty.