First of at least 3 posts on the subject (NOTE: Obl.Ang.List.content: hey, I'll bet the Maori have an interesting Gumbo recipe!):

GUMBO:
Part 1. Introduction:

Gumbo, while a widely known dish, is personal. If you ask a Cajun for his/her gumbo recipe, it will come to you orally, and not on paper. While most good cooks where I come from more or less keep recipes written down somewhere, they’re usually just a list of ingredients with little in the way of directions, and gumbos are more malleable - and almost archtypal, thus no need for a written recipe.

Gumbos all start out the same - with a roux; absent that and you’re just making soup or Courtboullion (another delicious subject possibly worthy of our attention) or something else, but, absent the roux, gumbo it just ain’t gonna be. Rouxs are discussed elsewhere, but generally a roux is made with equal or nearly equal parts of flour and fat. The fat can be shortening, lard, butter, margarine or vegetable oil. I suppose you could make a roux using olive oil, but I wouldn’t risk it, I don't think it gets hot enough to cook the flour properly. You simply heat the fat and slowly add the flour - stirring constantly and taking care not to burn the flour. The entire process takes 12-15 minutes. The degree of doneness depends on the color - the darker the more done. Rouxs can also be made in the microwave and a dry roux can be made in the oven. None is as satisfactory as a roux stirred up in a cast iron skillet. Add your vegetables just befor the desired degree of color desired is achieved, as this will stop the cooking. If adding water, add warm or hot water so as not to “curdle” the mixture.

After that, there are a number of major departure points, depending on what ingredients are at hand, and more importantly the individual’s preference, family/community tradition and the region of the Gulf Coast from which one hails.

Some of these principal points of departure include:
fish, game, fowl, or both?
shellfish?
sausage or not?
trinity (chopped onion, celery & bell pepper) or just onion?
tomatoes?
okra or file?
if okra, fresh or smothered?

Some folks get really adamant over what should or should not be in gumbo. They don’t understand the dish. After you make a roux, your gumbo is made with whatever you have at hand.

Gumbos are served over cooked white rice, often prepared ahead - almost always better the second day than the first, and can be frozen. If you’re making an oyster gumbo, remove the leftover oysters before freezing for obvious reasons - fresh oysters can be added when reheating. Also file gumbo should be eaten fresh and not allowed to boil, as the sassafras’ thickening power breaks down with boiling and will eventually impart a bitter taste as leftovers.

Coming separately are recipes for two principal types of gumbo: the first a true seafood gumbo, and a second “other” okra gumbo made with sausage and game meat. Any variation or combination on the theme is generally acceptable. There are recipes for duck and crawfish gumbos, vegetarian gumbos (blech!), and I have even seen a recipe for a creole gumbo made with red wine and beef, sort of a Beef Bourguignonne derivitave that should be delicious, I just haven’t eaten it.