Mass marketer General Mills Inc. is carving out a niche in gluten-free food after realizing it could reach eager customers without costly ad campaigns.

The company's Betty Crocker brand is rolling out gluten-free mixes for cookies, brownies and cakes. The mixes are the first gluten-free offering from a major, mainstream brand in the cake-mix aisle. Gluten is a key protein in wheat, but many people react badly to it.

Ann...