My gravy turned to jello overnight. How can I make it last for leftovers?
I think you probably used too much flour for the amount of liquid in your gravy — instead of gravy, you made pudding. You might be able to thin it down by whisking in some additional liquid such as milk or water before reheating it.

However, next time use half (or even less) percentage of flour to liquid, and you'll have better results.