I discard used marinade. If you do use it, you should boil the marinade at least a minute or 2 to remove the bacteria from the meat.
A lot of marinades are salty, and might make a salty gravy. If you do boil it, you can mix cornstarch with cold water, dissolve it completely. Flour works, too, but it tends to clump on me...I've had luck with flour if I add some of the hot gravy into the dissolved flour, mix, then back into the gravy pan.