Freeze grapes until firm, 1 hour. Meanwhile, in a small saucepan, stir together sugar and lime juice; bring to a boil. Simmer until sugar has dissolved and liquid is syrupy, about 2 minutes. Let cool completely, 30 minutes.
In a punch bowl or pitcher, stir together lime syrup, club soda, frozen grapes, and (if using) gin. Serve immediately.