Gin can be dry, piney, aromatic or any other style a distiller chooses. Go for a floral-tasting gin for this fruity slushie.
In a saucepan, cook strawberries, rhubarb, 1/4 cup water, and sugar over medium-high heat until fruit has softened, about 5 minutes, smashing fruit with a potato masher. Strain through a fine-mesh sieve, pressing as much liquid out as possible. Discard seeds. Cool liquid.
Combine juice, gin, 3 tablespoons water, and lemon juice in a quart-size zip-top bag. Seal bag and freeze overnight. To serve, massage the bag to break up mixture. Tip slushie into a glass. Add garnish and serve immediately.
Make a bigger batch in a gallon-size zip-top bag for serving at a party.