Pat dry the chicken breasts with a paper towel. Using a sharp knife, cut them into nugget-sized pieces of 1-inch. Transfer them inside a zip bag, pour the dill pickle juice, and seal.
Marinate:
Place the zip bag in the fridge for 45 minutes to up to 1 hour.
Bread the nuggets:
In a medium bowl, large enough to fit the nuggets, whisk the egg. Add the milk, combine, and set aside.
In a gallon-sized zip bag, combine the gluten-free flour and the seasonings. Set aside.
Drain the pickle juice out of the zip bag and place the nuggets in the egg and milk mixture. Toss them around the bowl with a fork to coat them with the egg and milk liquid.
With the same fork or using kitchen tongs, transfer all the nuggets from the bowl into the zip bag containing the seasoned flour. Seal the bag and shake it to coat the nuggets well.
Cook the nuggets:
Pour enough oil into a frying pan to fill the pan about ¾" high. Heat the oil over medium heat until it reaches 375F. You can check the temperature with a meat thermometer. If you let the oil get too hot, the breading might brown or burn too quickly before the nuggets are finished cooking.
With kitchen tongs, transfer the nuggets from the flour bag, shaking off any excess flour, to the hot oil in the pan. Make sure there's about 1" of space between them. Cook the nuggets for 3 to 5 minutes, using the tongs to flip them over halfway through cooking if there's any side exposed.
Remove the nuggets from the oil onto a paper towel-lined plate to absorb any excess oil. Repeat the process with the remaining nuggets.
Preheat the air fryer to 400F for 5 minutes. Use kitchen tongs to remove the coated nuggets from the bag onto a few paper plates (or a regular plate). Spray both sides of all the nuggets with cooking spray.
Place the nuggets in the air fryer and cook them for 12 minutes, making sure to shake the basket or flip them over halfway through cooking to ensure the nuggets get golden brown on all sides. Larger nuggets may need up to 15 minutes. They're cooked when the largest piece's internal temperature reaches 165F.