C.P.
I WOULD run to the store for cornstarch if I were you. It's SO much easier.
Don't add anymore flour unless you want it to taste like flour.
Here's what you need: Cornstarch, little bowl, COLD water, a fork.
Take 1/2 to 1 Tablespoon (depending on how much gravy you're making) of cornstarch in a small bowl and dribble in cold water while mixing with a fork. Add enough water to get a runny, smooth liquid with the consistency of cream. Mix it with the fork in your little bowl until there are no more lumps.
Bring the heat of your gravy up. When you see it start to bubble (and not before!), SLOWLY pour the cornstarch mixture into the gravy while whisking. Once you've mixed the cornstarch thoroughly into the gravy, give it about 20 more seconds at that higher temperature, and then reduce the heat to low. The cornstarch will thicken the gravy as it cools down.
ETA: While you're at the store, if all else fails, pick up some Heinz turkey gravy. You can add your own drippings to it and it really does taste very good. I always buy a few jars just in case my gravy needs a little boost. :o)