In a small bowl, combine chili powder, cumin, oregano, cayenne, salt and pepper. Season pork loin with spice mixture, rubbing in thoroughly on all sides.
Heat olive oil in a Dutch oven or large pot over medium high heat. Add pork loin and sear both sides until browned, about 3-4 minutes per side.
Place garlic, onions, pork loin and 1 cup water into a slow cooker. Cover and cook on low heat for 8 hours or high for 4-5 hours.
Remove pork loin from the slow cooker and shred the meat before returning to the pot with the juices; season with salt and pepper, to taste, if needed. Cover and keep warm for an additional 30 minutes.
In a large bowl, combine cabbage and carrots. In a large glass measuring cup or another bowl, whisk together Greek yogurt, mayonnaise, apple cider vinegar and sugar, to taste. Pour mixture over cabbage slaw, and stir using a rubber spatula until well combined. Cover and place in the refrigerator for at least one hour.
To assemble, top each pita bread with pulled pork, slaw, tomatoes and avocado. Serve immediately.