Real Cajun Gumbo

1 cup plus 1 tablespoon grape seed oil
6 boneless skinless chicken thighs
Kosher salt to taste
1 1/2 pounds Cajun-style andouille sausage slice into coins about 1/2 inch thick
1 cup all-purpose flour
2 large yellow onions cut into 1/4-inch dice
2 green bell peppers cut into 1/4-inch dice
4 large celery ribs cut into 1/4-inch dice
8 medium cloves garlic minced
1/4 teaspoon cayenne pepper
Freshly ground black pepper to taste
1 1/2 quarts 1.4L homemade brown or white chicken stock or store-bought low-sodium chicken stock
2 dried bay leaves
2 large sprigs fresh thyme
1 pound fresh okra (caps trimmed, pods cut crosswise 1/2 inch thick) - a package of frozen precut works just fine as well (optional)
1/2 teaspoon filé powder plus more as needed for serving (optional; see notes)
Warm rice thinly sliced scallions, and hot sauce, for serving