Combine the sugar and 1/2 cup water in a small saucepan, bring to a boil and cook until the sugar dissolves. Transfer to a container with a lid and refrigerate until cold, about 30 minutes.
Combine the lemon juice, grapefruit juice, gin, limoncello and tea in a pitcher. Add the sugar syrup to taste and stir to combine. Cover and chill for at least 2 hours and up to 24 hours.
Put ice cubes in 6 to 8 pint-size Mason jars and fill three-quarters of the way with the lemon tea mixture. Top with a splash of club soda, if you want your drink a little fizzy. Garnish with a lemon slice.