My fave drinks are white wine and vodka and diet coke (not all in the same glass!). I have a drink or two in the evenings some days and I haven't noticed any difference in my weight loss and it hasn't knocked me out of ketosis. So I'm sure a G&T (as long as its slimline tonic) will be fine. If you fancy a treat and it keeps you on the straight and narrow, why not? Better than a slice of cake!