Pour 1 cup of grapefruit juice and 1 cup of lemon juice together in a bowl.

Pour the gelatin onto the juice, letting it sit for 5 minutes before stirring it in.

Pour the leftover grapefruit juice into a small saucepan. Add honey and bring to a low simmer. Stir until the honey has full dissolved.

Pour the hot juice with honey into the gelatin bowl, whisking and mixing well for an even consistency.

Pour in the sparkling lemon water, whisking until the mixture is smooth and fully combines.

Divide the mixture into containers and refrigerate 4 hours until the jelly is set.