Gravy is simple in execution and content. If you cook anything in a pan, you can make gravy when you are finished. If you cook chicken and take it out of the pan, there will be pieces of chicken or skin left in the pan with some oil. You want only about 2 tablespoons of oil, so pour off any excess. With your pan on medium-medium-high, add an equal amount of flour to the oil and whisk them together letting the flour soak up the oil. Let it cook for two minutes or so to cook off the flour taste (whatever that means, just cook it a bit!).
Now you have some nasty-looking hot paste in your pan, so it is time to add the liquid. For a creamy white gravy, add milk. For a clearer gravy, add stock--chicken for chicken, beef for beef. Keep your pan pretty hot; the hotter the pan, the faster the gravy will thicken up, but don't forget to stir fast with your whisk.
Stand there whisking until it is the thickness that you want. Add more liquid if it is too thick, but adding flour is going to leave that weird taste, so add your liquid sparingly so you don't get in that position. Ballpark is about a cup of liquid for each tablespoon of flour. Add salt and pepper. Caution: with white gravy add pepper at the table or else you will have unappetizing gray gravy ;(