Mix 2-1/2 cups lime juice, 1-1/2 cups gin, and 1/2 cup sugar in a 2-qt. pitcher. Chill for several hours.
Dip rims of glasses in lime juice and coat with sugar.
Measure ½ cup Freixenet Cava into cocktail shaker filled with ice.
Add ½ cup of lime juice-and-gin mixture.
Shake and strain into 2 glasses.
Add 2 to 3 drops grenadine to each.