Place sugar in a bowl cover with boiling water and whisk. Sprinkle gelatin over the solution, and whisk lightly to incorporate. Let gelatin mixture cool to room temperature.
Pour the gelatin mixture into a large pitcher. Pour out an 8 ounce glass of the cava and pour the rest of the cava, on an angle, into the pitcher to minimize foam. Stir gently with the whisk. Use the reserved 8 ounces of cava for another purpose.
Pour into 6 wine glasses and refrigerate immediately. Refrigerate until set, at least 2 hours.