Duck gumbo is one of the best ways to use your ducks. If you (and everyone eating with you) like duck flavor you can use all duck and sausage. If you want to cut the duck flavor a little you can use chicken, duck, sausage. As folks have said, equal parts oil and flour to make a good dark roux. I typically use 1 cup each in a cast iron skillet over medium or higher heat and stir continuosly - do not burn! Stop at a melted chocolate color if you want a dark gumbo or a peanut butter if you want it lighter. I prefer dark. I let it cool then skim the oil off the top once it separates.
Boil your ducks/chicken in water or a combination of water and chicken broth (my preference) with some vegs/seasoning thrown in. Get your onion, celery, bell pepper, garlic sauteed in your gumbo pot until slightly soft then add your strained stock from the boiling pot. Once you get it up to a boil I start to stir in the roux from the cast iron skillet until you get the color you are looking for. I typically use ~ 12 cups of stock per 1 cup of roux (as in 1 cup flour/1 cup oil roux) but this isn't exact - just get to your desired color. Duck stock is darker than chicken stock so it darkens up fairly easily. Get this to a boil then get your deboned duck/chicken and andouille sausage in the pot, add seasonings, and let simmer until the roux taste cooks off. Throw in some green onions at the end of the boil then turn it off. Add file if desired, but only add it to the end of the boil as it can make the gumbo slimy if it cooks in for a while. A lot of people add it at the table rather than to the pot for this reason. Gumbo is always better the next day so if you are cooking for a group or event try to make it the day before then reheat the next day.
Now I'm hungry!