Chop up the rhubarb stalks into 2 ½ cm (1 inch) pieces. Add them to a large clean jar and pour over the granulated sugar. Add on the lid, then give the jar a shake to coat the rhubarb in sugar.
Leave the rhubarb to sit in the sugar overnight. It will become fragrant and juicy.
The next day, pour the gin over the rhubarb and sugar and close the lid.
Leave it in a cool dark place to sit and infuse for at least 4 days before sampling, gently shaking the jar every 2 days. For a stronger flavor, leave it to infuse for up to 4 weeks.
Once it's infused to your liking, strain out the rhubarb. The rhubarb gin will last in a sealed bottle or jar for up to 12 months in a cool dark place. Over time the deep pink color will fade a bit.
Serve with tonic, soda water, or your favorite mixer. Keep the mixer fairly neutral, so you don't lose the taste of the rhubarb.