For the ruby red grapefruit cordial:
In a saucepan, combine the grapefruit juice, lime juice, sugar and makrut lime leaf (if using) and gently bring to a boil- stirring occasionally.
When the syrup just starts to boil, add the zest and remove from heat.
When the syrup has cooled to room temperature, strain out the solids, bottle and store in the refrigerator for several weeks.
For the cocktail:
Combine the gin and grapefruit cordial in a cocktail shaker with ice.
Shake until ice cold and strain into a chilled cocktail glass.
Garnish with a thin, quartered slice of ruby red grapefruit or lime leaf.