In a small saucepan, combine the sugar, thyme, and 1 cup water. Boil, stirring occasionally, until the sugar dissolves, 3 to 5 minutes; chill. Remove the thyme sprigs.
In a large punch bowl, combine the thyme syrup, orange juice, and gin; chill for up to 12 hours. Just before serving, add the club soda and additional thyme sprigs.