Pour the tequila, vermouth, gentian liqueur and water into a resealable bottle (flip top or screw cap preferred). Tightly seal and freeze overnight.
To serve, add a large ice cube to a rocks glass. Pour 3 ¾ ounces Tequila Negroni into the glass. Holding the orange peel by its long edges and skin facing down into the glass, pinch the peel to express the citrus oils; drop the orange peel into the glass and serve immediately.
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