There are bottled and pouched gravies for dogs. I remember some time ago there was a bottled gravy to pour on cat food but I don't remember the name of it. I haven't seen it in pet stores in a long time so it may no longer be available. The ingredients in any commerical gravy product for pets are likely very poor quality, full of artificial flavors and chemicals and straches.
I'm not familiar with a way to make homemade gravy. Have you tried blending a can of cat food in a blender to turn it into liquid and pouring that over food as "gravy"? Adding water to canned food results in a sort of "broth" which many cats like.