You can start this from frozen pork without defrosting, but you may need to add extra cook time. If starting from frozen, wait at least an hour into the cooking time before adding the salt, pepper, mustard, and brown sugar so that it doesn't all just come off as the outside defrosts.
The cooking juices are slightly sweet and very flavorful for serving, but if you prefer, you can discard the juices after shredding and removing the meat (leaving behind any remaining garlic and small bits of meat) and make gravy instead. To do this, you will need 2 tablespoons butter, 2 tablespoons gluten free flour blend without xanthin gum (I like Bob's Red Mill All Purpose Gluten Free Flour), and 2.5 cups of chicken broth. Using the same pot, melt butter over medium heat and whisk in flour and allow it to bubble, then slowly pour in chicken broth, whisking as you go until the gravy reaches your desired consistency. Taste and add salt, pepper, and garlic powder to taste.