Xanthan Gum in sauce/gravy
The flour in this recipe is for thickening the sauce.  You can use xanthan for this purpose, but in very small amounts...0.25 to 0.75% for a thin to medium running sauce.  As you increase the amount, xanthan will make your sauce take on an unpleasant, mucous-like texture.

This is most easily calculated in weight (and metric).  I'll have a go, and convert it back:

You have 2 cups of broth = about 480 grams + some other liquid.  I would suggest 500 grams is a reasonable estimate.   .25% of 500 grams = 1.25 grams of xanthan (start on the low end.  You can always add more).  There are about 4 grams in a teaspoon.  I would suggest 1/4 teaspoon of xanthan as a reasonable starting point.