a few cups of Beef/chicken broth (i usually will cook my meat IN broth, so i just use that)
Pour the broth into a saucepan, and bring to a boil. In a SEPARATE bowl, mix the cornstarch and COLD water/milk in a 2 to 1 ratio (2 parts liquid, 1 part cornstarch) and dissolve together. while whisking, pour the cornstarch mixture into the broth and turn the heat down to a simmer. keep whisking as the mixture thickens. add garlic and onion powder (and any other seasonings you want) and keep on heat until if reaches the thickness you want. It you want a thicker gravy, make more of the cornstarch mixture and add it it slowly, if you want a thinner gravy, add more broth.
Seriously you will never make gravy from a packet again!