Bring water almost to a boil in a large pot. Remove from heat, add green tea bags and steep for 4-5 minutes.
Once the tea is steeped, add sweetener if desired. [You can also wait to add sugar until serving if other people will be drinking it too. I recommend 1/4 cup of honey for the whole pot or 1/2 tbsp per serving. Monkfruit sweetener or coconut sugar would also work well, especially for muddling the mint in individual glasses.]
Allow tea to cool, then add grapefruit juice and mix.
Add mint leaves to bottom of glasses or a pitcher. If desired, add your sweetener of choice at this point and muddle the mint leaves with sweetener.
Add ice cubes to the glass and pour green tea over top of muddled mint. Top off with sparkling water before serving.
I didn't calculate the nutrition info for this drink because it completely depends on your choice of sweetener and how much you add to match your tastes. Aside from the sweetener, the only calories come from grapefruit juice.
I recommend the following amounts of sweetener for the entire recipe: