Is taking 1 gin and tonic per night bad?

Not usually: unless you have a history of alcohol abuse or dependency, in which case it will very likely not stay at 1 drink. If you are using a shot glass and are having 1.5 oz. of gin per night, that is within safe drinking limits for women. If you are using it to help sleep, however, be aware that alcohol commonly causes a "rebound" where people wake up 3-4 hours later&may have trouble sleeping.