Prepare the ice cubes: Add a few fresh thyme leaves and grapefruit juice to each cell of an ice cube tray. Freeze for 2-4 hours, or until fully frozen.
1 cup grapefruit juice, fresh thyme leaves
Make the punch: In a pitcher or punch bowl, add the gin and grapefruit juice, and mix together. Place in the refrigerator to chill.
2 cups gin, 4 cups grapefruit juice
Add the sparkling wine: When ready to serve, pour on the sparkling wine and gently stir everything together. Add in half the grapefruit ice cubes, reserving the other half to place in individual glasses.