Smell Something Burning. Course of action?
So with gravy, you are talking about a scorched taste. Once you smell it, it is probably too late to salvage it. If you want to try, the best course of action is to remove it from the heat (obviously) and carefully ladle off as much of the top of the pan as you can, leaving the scorched part on the bottom. Then taste what you ladled off and see if it is really edible. In my experience, people try to convince themselves that the scorched flavor isn't really there, but it is, and once it is, all is lost.