Pick the grapes from the stems and place them in a single layer on a rimmed cookie sheet. Put the grapes into the freezer and freeze for at least 3 hours or overnight. Once frozen, the grapes can be put into a plastic freezer bag.
When ready to serve, in a large pitcher stir together the lemon-lime soda, gin, and lime and lemon juice. Fill a glass with frozen grapes and pour over the cocktail mix. Garnish with slices of lemon and lime if desired.