You can use xanthan gum as a substitute for guar gum and each can often be used in place of the other.  If using xanthan gum in place of guar gum, decrease the amount used by 50%. It is also a good agar agar substitute.  Xanthan gum won’t set up the same as agar agar, but it will thicken the recipe and keep ingredients from separating.  Use it in a 1:1 ratio with agar agar.