Combine the cucumber slices and sugar in a resealable plastic bag. Press out the air, seal the bag, and toss gently to coat the cucumbers in the sugar. Set aside at room temperature for 4 hours. Add a little water to the bag to help dissolve more of the sugar, then strain out the cucumber slices, reserving the syrup. Store the syrup in an airtight container in the refrigerator.
Stir together the tonic water, lemon juice, and sugar in a large mixing glass until the sugar has dissolved. Add the lemon zest strips and use a muddler to push down on and twist the zest at the bottom of the mixing glass, muddling for about 1 minute. Allow the mixture to rest for 10 minutes before removing the zest. Store the lemon tonic water in an airtight container in the refrigerator.
Combine the salt, ground dried hibiscus flowers, and water in a mortar, using a pestle to crush and grind them together. Once combined and the salt turns a pinkish hue, spread it out in as thin a layer as possible on a baking sheet lined with parchment paper and allow the salt to dry out completely overnight. Break the sheet of salt apart into small pieces and store in an airtight container at room temperature.
Chill the 4 serving glasses by placing them in a freezer or filling them with ice cubes. Add the gin and cucumber syrup to a cocktail shaker. Fill the shaker with crushed ice, place the cap on the shaker, and shake it vigorously for 10 seconds.
Strain the mixture into a large mixing glass. Add the lemon tonic water and stir to combine. Distribute between the 4 serving glasses and garnish each one with 3 pink peppercorns and a pinch of the wild hibiscus salt.