When gravy has gone bad it will usually have a bad smell to it and may also have some mold in the gravy.
If the gravy has been out at room temperature for more than 2 hours then it's likely bad and should be thrown out.
You should always refrigerate any leftover gravy as soon as possible and well before 2 hours if possible to keep bacteria from growing on the gravy.
The gravy should last in the refrigerator for 4 to 5 days but if it smells bad or has a bad taste then throw it out.
Usually you can see that the gravy has changed colors and begins to smell sour or other bad smells coming from it.
If so throw it out.
If your gravy contains chicken then it will also have a bad smell to it that would make you vomit.