Wash grapes thoroughly. Remove stems. Pat dry.
Place dry grapes in large bowl.
Zest limes directly over grapes.
Add fresh lime juice.
Add sugar and salt if using.
Toss gently until evenly coated.
Line baking sheet with parchment or silicone mat.
Spread grapes in single layer. Do not let them touch.
Place in freezer.
Freeze at least 2 hours until completely frozen.
Transfer to freezer-safe bag or container. Keeps up to 3 months.
Serve frozen. Garnish with lime zest or mint. Eat immediately.