This drink is based on nimbu pani, a lemon- or limeade popular in India. A little orange-blossom water, while not necessary, gives it a delicate fragrance.
Stir together lime juice, gin, orange-blossom water, simple syrup, and salt in a pitcher until thoroughly combined. Slowly add seltzer and gently stir until combined. Fill 6 glasses with ice. Divide cocktail among glasses and garnish with lime rounds. Serve immediately.