In a large bowl, whisk milk, vinegar, 2 teaspoons salt, and 1 teaspoon pepper. Add chicken pieces, cover, and refrigerate for at least 2 hours (up to overnight).
In a resealable bag or shallow dish, combine gluten-free flour, cornstarch, onion powder, garlic powder, remaining 2 teaspoons salt, paprika, thyme, oregano, and basil.
Heat oil in a 10-inch skillet to 350°F (test with a bit of flour mixture: it should sizzle but not smoke).
Working in batches, coat chicken in the flour mixture, shaking off excess. Fry until golden and cooked through (about 8-10 minutes per side). Transfer to a wire rack to drain.
Serve warm with optional dipping sauce or a simple side salad.