When a cat suddenly stops eating food they once loved, especially an elderly cat it's often the sign of a serious problem. The fact that he's licking up the gravy is a somewhat good sign, it shows that he's hungry and trying to consume something. CosmicGiggles sounds like she might be on the right track with it being a dental issue. I had an elderly cat once that had dental issues and she did more or less the same thing as your cat so my wife and I started running her food thru a food processor to chop the food up even finer then it already was as well as mix it in good with the gravy. This worked for a while but then a few months later she wouldn't eat at all so we took her to the vet who said her kidney's had shut down and there was much he could do other then euthanize her rather they let her suffer the inevitable painful death that she would soon face. I hope that's not the case for your cat but if he's above 13 years old that could be the direction he's headed in. I say 13 because years ago that was considered the typical lifespan of a cat. More recently though with people keeping their cat's indoors and better foods cat's often live up to around 20 years but it also depends on the breed and how good the genetics are.
