My mother has a severe gluten allergy, so need alternatives to flour as a thickening agent.

Corn Starch

1/2 Tablespoon corn starch can be used instead of 1 Tablespoon of flour. Mix the corn starch with enough cold water to be smooth. Instead of adding this to the pan before the liquid as we would when using flour, add the liquid first then add the corn starch mixture. Bring to a slow boil to thicken.

Guar Gum

A little of this goes a long way, so add a teaspoon at a time and mix with your liquid until your desired consistency is reached.