how do you fix burnt gravy?
If it's just scorched on the bottom of the pan, and you haven't mixed the burnt material into the rest of the gravy, you can try just pouring the good gravy into another pan.

If you've tried to stir it and scrape the burnt stuff off the bottom, there's not much you can do. The human palette can detect very small quantities of burnt flavours, so you won't be able to mask it.