We won't pretend it happens often, but Christmas is one of the few occasions where you might end up having to transport gravy. If that happens, your secret weapon is a simple thermos. It will keep the gravy warm until it's ready to serve.
Pre-warm the thermos with some hot water, then ladle your gravy, once it's hot and smooth, into it. If it works for your coffee, it will work for the gravy -- when you get back to it, the gravy will still be warm and smooth, even if that's hours later.
The video above from America's Test Kitchen explains why this works and why your gravy doesn't end up coagulating.
What Smart Cooks Do on Thanksgiving: How to Keep Gravy Warm for Hours (Without a Gravy Warmer) [America's Test Kitchen (YouTube)]