Sometimes it's difficult to find the gluten free flours that a recipe calls for, or sometimes you are just simply out of that particular flour that you need so you have to make a substitute. When substituting flour (of any kind) it is always best to measure by weight and not by cups. That way you get a more consistent result.
Here is a good explanation as to why this is important:
And here is where you can find a table on the weights of many gluten free flours (click the link at the bottom):