Susie, we love that you're so committed to making a cake for your friend with special dietary needs! We'd recommend guar gum in place of xanthan gum for your friend. You can use the exact same amounts of guar gum that you would use for xanthan gum. We haven't tried it in all our recipes, but in our experience it's a very good substitute. Happy baking! Kat@KAF

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