Keep in mind that I am certainly not a Chef.
I'm sure they have some name for doing this stuff,
but I have no idea what it is. I only know how to do it.

Adding layers to Gumbo isn't necessary at all to get excellent Gumbo.
You can add all ingredients into the pot at once and get a wonderful gumbo.
But layers open the door to many distinct flavors and combinations not possible with and "all at once" approach.

Layering stew type dishes simply means adding ingredients during different times in the cooking process, sometimes the same ingredient several times,
to achieve different textures and flavors. Onions, celery, bell pepper, and other vegetables can be layered  several times into  Gumbo to present a multitude of flavor and texture variations  from each of them.

Meats can also be layered, and sea food should be the final layer in a gumbo because of the minimal time required to cook sea food and fish. Also seafood quickly will loose its flavor if cooked too long.

In most of my gumbos, I use three layers of aromatics, two layers of spices, and usually one layer of meat with sea food added at the end, if I wish to use it.

I start boiling the ducks or other meat with a stock of aromatics cut fine so as to release all their oils and then become soft and barely negligible texture wise. Then after the meat is cooked to tender, I remove it and go with the second and main portion of the veggies. Then toward the end of the cooking, to add both crunchy texture and fresh taste, I add yet another small layer that is barely cooked and still holding most of its flavor within the actual piece, along with another dose of spices such as basil and thyme. I also lace my rice with fresh chopped uncooked chives and parsley, which give the gumbo a distinct and very fresh flavor that cuts through the heaviness of gumbo and allows for other flavors to catch your palette.

Layering can produce more distinct flavors in gumbo than you can cook in a lifetime, combined with the individuality of the flavor of each roux, it produces a pot of gumbo each time that is different yet delicious.

One trick that I discovered quite by accident, is starting a stock with a couple ducks or some other meat, (no seafood) and aromatics, and cooking it down to a simmering thick layer in the pot bottom, and then adding water and letting it simmer down yet again to almost nothing to attain a roasted baked sort of flavor in the mix. This cooks the meat to mush, which you can then take from the bone and puree into a paste, adding some of the stock if it gets too sticky.

(You can also use this stuff to make all kinds of nifty jet meat spreads for crackers and such)

This mixture added to a NEW pot of gumbo at the beginning phase of a new pot, as an addition to the original stock, is unsurpassed in adding thickness and unique flavor to the gumbo. But since it takes several hours each time to simmer it down, it turns a pot of gumbo into a two day affair. But for the discriminating palette, it is well worth the trouble. 
This gumbo is so rich that you may only be able to eat a bowl full at a sitting.
I has "umph" that no other stew type mix can deliver.
Adding smoked sausage into the original layer for puree gives the whole pot even more zing, with the individual pieces that go into the following layers giving a taste of the pure sausage to go with the overall flavor.
It's quite tantalizing to say the least.

You can experiment with your favorites and find success quite easily,
but good luck duplicating the flavor.
I have made pots of Gumbo from which the first spoonful made me immediately think I can NEVER get this to taste like this again.....

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