Sadie Scheffer started baking gluten-free bread to impress a crush who would become her husband. It worked, and it impressed a lot more people, too. With her gluten-free sourdough, Ms. Scheffer was able to start a small wholesale bakery, Bread SRSLY, Berkeley, Calif. In the past eight years, the bakery has managed its own production, outsourced to a co-manufacturer and brought production back in-house. But its commitment to gluten-free, sourdough has remained consistent.