When a food group, or type of food is removed from the diet we naturally seek to substitute it for something that is familiar to that that eliminated. e.g. rice or soy milk for cows milk, or gluten free versions of bread, cakes, etc.
My preference is to seek alternatives not substitutes.
When first starting out, substituting is easier. We are creatures of habit more than we are willing to admit so suddenly stopping not just gluten containing foods but the nature of the food itself is difficult. Stopping all bread, crackers pasta, cake, biscuits, pastry, etc is difficult, though a very good thing to do!