I used to use xanthan gum. Since I have been using almond and coconut flours, I generally don't need to add any gums to the recipe.
I think that you can sub one for the other, but I know that guar gum has the reputation of causing indigestion/loose stools in some people, and since I have plenty of the "D" without it, I try to avoid guar gum.
Xanthan gum is produced using some sort of corn product, and guar gum is produced using a legume product, so people trying to avoid corn or GMO foods should probably avoid xanthan gum, because corn is usually GMO.
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