Cream gravy isn't always made with cream, it can also be made with milk. A basic cream gravy uses the drippings in the pan from the meat, to which you add flour to make a roux. You cook the roux for a bit, then add the cream/milk and whatever seasonings you'd like to add if any, and whisk it well to thicken.
Like marrec says, a good roux is important for gravy, and depending on the type of gravy you're making, you might cook it briefly, or until it's nearly burnt to alter the flavor. Cream gravies don't need much, beef gravies you want to push your luck.
And any lumps in the gravy should be bits of meat, not flour.
Also, to get back on topic, in my experience as a US citizen, our use of the term "open face sandwich" refers to the ones that do have gravy slathered all over them, so that eating them like a sandwich you hold in your hand would be impossibly messy. Both pieces of bread are there underneath though. Roast beef, pot roast, and turkey open face sandwiches are heavenly with some mashed potatoes.
