Sprinkle gelatin over cold water in a small bowl; set aside. Combine lime juice and egg yolks in a small heavy saucepan; cook over medium-low heat 10 minutes or until slightly thick and very hot (180°), stirring constantly (do not boil). Add softened gelatin to lime juice mixture; cook 1 minute, stirring until the gelatin dissolves. Place pan in a large ice-filled bowl; stir gelatin mixture 3 minutes or until mixture reaches room temperature (do not allow gelatin mixture to set). Strain gelatin mixture into a medium bowl; discard any solids. Gradually add milk, stirring with a whisk until blended (mixture will be very thick); spoon mixture into Graham Cracker Crust; spread evenly.
Beat egg whites, cream of tartar, and salt at high speed of a mixer until foamy. Gradually add sugar, 1 tablespoon at a time, beating until stiff peaks form. Spread evenly over filling, sealing to edge of crust.
Bake at 325° for 25 minutes; let cool 1 hour on a wire rack. Chill 3 hours or until set. Cut with a sharp knife dipped in hot water. Garnish with lime slices, if desired.