A roux is also the essential ingrediant to Gumbo. Various stages of roux can be made depending on the desired usage. For a gravy, a lighter roux is desired as it has a greater thickening power. For a Gumbo, a very dark roux should be made. To make a roux for gumbo, one should mix equal parts flour and vegetable oil in a skillet over medium high heat, just enough to get it to bubble. Cook for a while while constantly stirring. The roux is done when it is about the color of chocolate. The general rule of thumb is that a roux should take about the same amount of time as drinking 2 beers. I've found it takes about 45 minutes to make a proper gumbo roux. Do not leave it alone, or it will burn and give the gumbo a bitter taste.