Xantham gum cannot be used as a substitute for gum Arabic

Xanthan gum doesn't function as yeast/leavener; it mimics the function of gluten in flour. XG turns the dough gummy and sticky, which permits the dough to trap gas bubbles from the yeast(or other leavener), which in turn allows the dough to rise. Imagine if you will a large piece of bubble gum. That's the texture of wheat flour dough, and with xg, that will also be the approximate texture of your GF dough. 

Now onto the matter of xanthan gum and fermentation. XG is made by fermentation, it does not itself ferment. Many things are made by fermentation. In the case of XG, sugars are digested by bacteria, and they excrete xanthan gum. Yummy, huh? Another example is vanilla. Vanilla flavor is created by the fermentation of vanilla bean pods. But it doesn't itself ferment or create fermentation in cookery. Many other things are created by fermentation. 

While some people think that depends what for, but not really.  Xanthan gum is great for forming viscous solutions. Gum Arabic stabilizes emulsions and makes films but isn't that viscous.  Xanthan is a large stiff polysaccharide but gum Arabic has some protein groups that makes it amphipillic.

Xantham gum cannot be used as a substitute for gum Arabic. But gum arabic can be used as a substitute for xanthan gum. Xanthan gum may cause allergic reactions, especially those who have corn allergy in some people. For the case, you can substitute for xanthan gum in a specific amount. Gum arabic is one of the alternatives. It a natural food stabilizer, which is derived from the sap of various species of acacia tree. The low viscosity, high emulsification and adhesion properties make gum arabic an excellent ingredient in foods.