To make a thick creamy gravy the key is to stir the entire time not letting it get lumpy, and the easiest way to make gravy is to buy instant gravy mix, it comes out perfect every time.
The trick for thick and not lumpy gravy is to take your thickener, either flour or cornstarch, and mix it with a little bit of water (aim for toothpaste consistency). Put this on the heat and slowly add more broth until you reach your preferred thickness.