Grate 1 tablespoon peel and ream 1 cup juice from lemons; set aside.
Combine gelatin and 1/4 cup cold water; set aside until gelatin is softened, about 5 minutes.
In a 2- to 3-quart pan over high heat, bring sugar and 1 cup water to a boil. Reduce heat and simmer until sugar is dissolved, about 1 minute.
Stir in gelatin, lemon peel and juice, and milk (mixture may look curdled). Place pan in a bowl of ice water and stir often until cold, 8 to 10 minutes.
Freeze mixture in an ice cream maker according to manufacturer's directions. (Or pour mixture into a metal 9- by 13-inch pan, cover airtight, and freeze until hard, about 3 hours. Then break into chunks with a spoon and whirl in a food processor until smooth.)
Serve sherbet, or freeze until firm enough to scoop, about 2 hours.