Guar gum is a stabilizer and thickener derived from the guar or clusterbean plant, which is native to India. Guar gum is often used as a gluten substitute in gluten-free baked goods, and it is found in foods such as ice cream, pudding and gravy. The website Practically Edible says it has more thickening power than cornstarch.
Add up the amounts of liquid used in the recipe. Convert this total to quarts; 1 quart equals 4 cups, 1 cup equals 16 tbsp. and 1 tbsp. equals 3 tsp. Francesca Berrini of Bob's Red Mill writes that for each quart of liquid in hot recipes, such as gravy, use between 1 and 3 tsp. of guar gum. For cold recipes like salad dressing, use 1 or 2 tsp. per quart.
Mix guar gum in thoroughly with any oil called for in a salad dressing recipe. Practically Edible notes you need only a small amount, less than 1/2 tsp. per 2 cups. Berrini recommends using a blender to mix the two together before proceeding with the rest of the recipe.
Multiply the amount of xanthan gum in a recipe by 1.5 if you plan to use guar gum as a substitute. RecipeTips.com says you can use guar gum in place of xanthan gum, but you must add 50 percent more guar gum than xanthan gum. Berrini says xanthan gum is formed from an organism that feeds off soy and corn, and that the risk to corn- and soy-allergic individuals is unknown. Substituting guar gum eliminates this risk.
Berrini notes that acidic foods like lemon can prevent guar gum from thickening properly.