Good for you to be a G&T fan! They do seem to have gone out of popularity with the yoof of today but they are still a part of the establishment in the City, London.
My two favourite London gins are also Bombay Sapphire and Tanqueray, with Gordon's coming in third. You should also try to obtain a bottle of Plymouth-quite a different style to London (and allegedly sampled by the Pilgrim fathers en-route)
It's true the tonic available in the US is less bitter and different than in the UK but the quality varies from brand to brand over here too.The best is made by Schweppes and one secret is always to open a fresh bottle.
When I lived stateside, I quite often added a couple of drops of Angostura bitters to the drink to enhance the aromatics.
I've travelled to India quite a bit and the quality of G&T over there varies tremendously. The tradition was that gin was added to the Malaria 'tonic' to make it more palatable and I would say in general the Indians serve it heavy on the gin with a smaller splash of tonic. With their sweet limes and sugary tonic water the end result is certainly different but not necessarily better. I guess it depends what mood you are in-long drink or stiff drink!
BTW, I also often drink in the summer what is called a 'Harry Lime'- just tonic water and Rose's Lime cordial (or equivalent). Quinine is a great drug for preventing muscle cramps when running or at night, but you need to drink a lot, even of the British stronger variety.