How do I add spice after cooking gravy?
Carefully! You can always add more, but you can't really remove spices once they are added.

If your spices are ground, you can just add them to the gravy and stir them in. Salt, you can add any time.

If your spices are whole, you can simmer them for a while in the gravy and them remove them. You can also toast them in a separate dry pan, grind them (see here), and then add them to your gravy. Again, carefully!