Oh, Big Dale, you've opened a topic that's driven me crazy for several years. I love turtle soup. Of course, I know that it's stretched with and flavored by other ingredients, so I feel I still have no idea what the turtle itself really tastes like.
You have rekindled my determination to get a better sense of it on its own.
Re your post, turtle soup and turtle sauce piquante and indeed most places I've eaten turtle have a common thread -- the inclusion of tomato. And sure enough, the gumbo recipe you linked includes tomato.
Did a little more searching. Found what may be a place to jump off for a "turtle gumbo" -- http://www.justgamerecipes.com/gam-misc0057.html
There's already flour in the recipe -- so a roux would clearly work, likely a medium dark roux. Even though it says to "saute" onions, it's your kitchen, so I'd say go ahead and cook your onion, plus celery, bell pepper and garlic in the roux.
The recipe calls for ham, so clearly a nice sausage would have no problem subbing and would not be out of place in gumbo.
I will say the notion of okra in this mix sounds like a bad horror movie, so I likely wouldn't go there.
The other question in terms of gumbo is whether the turtle will miss the lemon or the sherry?
I think I'm going to try to make this one from their site -- http://www.justgamerecipes.com/gam-0220645.html
And, just to finish off the thought -- yes, this has driven me crazy for a long time getting up the nerve to cook turtle myself, and even as we speak I'm trying to talk myself into doing it, I know of at least one restaurant that stopped serving turtle soup altogether back in the 1980s and switched to alligator. Perhaps find alligator gumbo recipes and substitute the turtle for the alligator meat.