In a small bowl, combine chili powder, cumin, oregano, garlic, salt and pepper. Rub spice mixture over pork loin.
Sprinkle onions around pork and pour beer over top. Cover and let cook on low for 8 hours or high for 4-5 hours.
After it is finished cooking, remove pork loin from slow cooker and shred meat. Return to slow cooker with juices and season with more salt and pepper, if needed. Cover and cook for additional 30 minutes on low.
Assemble gyros by topping each pita bread with pulled pork, coleslaw, tomatoes, then avocados. Serve immediately.