What should I do if gravy is too thin?

You can add in a reverse roux also called a Beurre manié which is just softened butter with flour, let it melt into your gravy and then bring the gravy up to a simmer. It should thicken.

Make ahead gravy does thicken on its own. In fact, I set aside stock to thin it down if it becomes too thick.

Hi Sauertea! If you continue cooking it over a medium-low heat, it should thicken up for you. If that doesn't work, you could whisk in a little bit more flour and see if that helps. My guess is that it'll thicken up once it cools, so it'll just be a matter of reheating it tomorrow. Hope that helps!