Run a grapefruit wedge around the rim of a highball glass, then dip the rim in coarse salt to coat evenly.
Fill the glass with ice cubes.
Add 45ml of gin directly over the ice.
Pour in 150ml of freshly squeezed grapefruit juice.
Stir gently but thoroughly to combine and chill.
Serve immediately.
The Salty Dog is essentially a Greyhound with a salted rim - that single addition transforms the drink, with the salt suppressing bitterness and enhancing the sweetness of the grapefruit. Always use freshly squeezed juice; carton juice will flatten the cocktail considerably.
My Bar Shelf reads your shelf and shows you every cocktail you can make from what you already own, including Salty Dog.