Gordon's Ultra Low Alcohol Gin & Tonic (£5) is just 0.5% ABV, and it's ready to drink so you don't even have to mess about with mixers. It's made from the same botanical blend as the original gin, so it's still recognisably that Gordon's taste.
Take your pick from two flavours — one with a hint of grapefruit, and one with a hint of lime, sold in packs of 4 x 250ml.