Who invented the griddle?

Gravy's been made since meat was first cooked and the juices collected, one way or another, and used to moisten and further flavour the meat.

All gravy made today follows this principle; we might add degrees of sophistication such as flavourings - onions, garlic, wine, thickeners such as flour, mashed potato, and so on - and perhaps an elegant gravy boat for serving, but it's still basically the juices left from cooking the meat.

I'm not counting gravy powder here, which is nothing at all like real gravy and a waste of time and money, since it takes just as long as making gravy from the pan juices.

If you want to know when was commercially-manufactured gravy powder first made, that's another question entirely.