I am a gin & tonic fan - I usually order a Bombay & Tonic, as that is by far my preferred gin, but if they are unlikely to have Bombay, e.g. a basic pub, then I will order a G&T and they will usually just pour the house brand (praying for Gordon's not Beefeater tho!) If the cost is the driver, then 'well brand' or 'bar rail' in N. America or 'house' in the UK will get you the basic gin they have.