Juice the oranges and grapefruit (or use non-concentrate juices instead)

Mix together in a large (ideally shallow) plastic container with gin and sugar as desired

Put in the freezer for 2-3 hours before using a spoon to mash the semi-frozen mixture so that it doesn't harden and become ice. Repeat as necessary one or twice more. The high fruit content will make it easier to scrape/scoop.