Add boiling water to the pan drippings and have it very hot before stirring in the flour. Stir the gravy constantly with a long wisk. I always add some Kitchen Bouquet Browning sauce. A cheat is if it gets lumpy, take your boat motor to it!!! (immersion blender)
We also use a tupperware gravy mixer and use cold water and flour mixed separately prior to adding to the gravy. Keep the pan drippings hot, stir in the slurry slowly.
This is one recipe the wife has mastered!
Flour type has never mattered. We always use AP.
Someone mentioned coffee, and that does have its place in some recipes. Mostly a red eye gravy for ham and biscuits. I call mine a brown-eye gravy.