I don't think any sane person would say gravy and curry are the same category.

Why? Culinarily*, curry and gravy are quite similar, and serve similar functions. Obviously they don't taste all that similar, but I don't think that really matters much when you consider the vast variety of flavors that curries come in.

And actually, now that I think of it, Japanese curries do share quite a few flavors with a Western meat-dripping-based gravy. In fact, I'm pretty sure the directions on the packaged* curry cubes I get from the Asian grocer refer to the curry sauce as "gravy". So yeah, actually, plenty of sane people put curry and gravy in the same category, for solid reasons.