Preheat oven to 350°F (175°C). Make bread crumbs by pulsing bread in a food processor, then bake on a sheet at 350°F for 10-12 minutes, stirring occasionally. Transfer to a bowl.
Whisk egg, mayonnaise, mustard, gluten-free flour, salt, pepper, garlic powder, and onion powder until smooth to make the wet batter.
Pat chicken dry, dip into the batter, then roll in breadcrumbs. Place on a prepared baking sheet and spray lightly with cooking spray.
Bake at 400°F (200°C) for 12-15 minutes until golden and cooked through (internal temp at least 165°F).
Garnish with fresh herbs if desired. Serve warm with your favorite dipping sauce.
Notes
For extra crispiness, use a wire rack during baking and spray the chicken lightly before baking.