The GAPS diet does not allow baking powder or raising agents of any kind apart from pure bicarbonate of soda, otherwise known in USA as baking soda. (Use sparingly for people who have very low stomach acid).
Egg whites (albumen) are especially good as a mechanical raising agent and for both soufflé and mousse the egg white and air structure formed by beating creates the texture of the product. So if you want a light, air filled texture then mechanical raising agents (like beaten eggs) are important.
GAPS Approved Raising Agents for Baking
Appropriate for Full GAPS and Stage 6 of the Intro Diet
Bicarbonate of soda is a pure leavening agent. It needs to be mixed with moisture and an acidic ingredient for the necessary chemical reaction to take place to make food rise. Because it needs an acid to create the rising quality, it is often used in recipes where there is already an acidic ingredient present, such as lemon juice, cocoa, yoghurt or honey. Baking Powder should be avoided as it contains additional non GAPS allowable ingredients.
People who have very low stomach acid may need to avoid Bicarbonate of soda for a while and egg whites may be used for an alternative raising agent. Simply whisk the egg whites into soft fluffy peaks before adding to the mix.