I think "gravy" refers to the sauces that are so prevalent. Like Rogan Josh has a sauces that is way tasty. Chicken Tikka Masala supposedly was created when some Brit said "where's muh gravy?" In the movies, you always see Tamil people with a big serving of rice, then a small bit of sauce, and massaging it around with the fingers. I can't see Brits eating that way, but I love the sauces, use them for all kinds of things, and can see why the sauces would make Indian food popular.