How do you properly thicken a gumbo?

Flour

How much oil and flour do you use at the beginning?

My roux make up is 3 cups flour to 1lb butter. This makes enough roux to thicken 5 quarts of stock.

roux and use chicken or turkey stock made with the same meat youre going to put in it

Yep, thicker roux and that means more flour.

You can also mix a few tablespoons of Corn Starch with a little bit of water and add it to the gumbo and that will thicken it up pretty easily.

Go a little at first and let it simmer a bit to see how much it thickens the gumbo. If not thick enough, do a little more until you get the consistency you want.

It will not change the flavor of the gumbo, just the gravy thickness.

easy answer is to use less liquid to begin with.
I like a dark roux and a thinner than gravy consistency. Nothing worse than a pasty gumbo which is a common result when using more flour in the roux

To each his own but I don’t understand this boards obsession with thick gumbos. Growing up in Lake Charles and Lafayette areas most families made thinner gumbos that what you normally see in restaurants in Baton Rouge and New Orleans. I remember reading a quote by Paul Prudhomme saying the best gumbos were of the thinner variety. My chicken and sausage gumbo is of the same consistency as Chris’s Poboys in Lafayette for those who have had it.

If you don’t want the floury taste use Xanthan gum. A little goes a long way. It comes in a powder