Combine 1 tablespoon sugar with chopped thyme in a shallow bowl. Rub the rim of a glass with the grapefruit wedge, then dip in the sugar mixture to coat.
Muddle more thyme leaves and 2 teaspoons sugar with gin in the glass. Stir in freshly squeezed grapefruit juice. Fill with ice and top with seltzer.
Garnish with a grapefruit slice and thyme sprig.