Grate cucumber with a cheese grater. Place grated cucumber into 3 large paper towels and squeeze over the sink to strain. Squeeze well to make sure all the water is out.
Add cucumber to bowl with remaining tatziki ingredients and mix well. Cover and chill until serving.
Place bullion, water, and spices in slow cooker. Mix with a spoon Then place beef in slow cooker and red onion.
Cook on low for 6-8 hours or on high for 4-5 hours.
Once cooked through, shred beef with two forks in large bowl. Pour some of the drippings over the beef to keep it moist and give it more flavor. Keep covered with aluminum foil until ready to serve.
Before serving, warm up pita bread. We do this in the microwave in a tortilla warmer which keeps them soft. If you don't have one you can warm them up and keep them in aluminum foil until you're ready to eat.
To assemble Gyros, layer each pita with beef, lettuce, red onion, tomato, and lots of tatziki. Top with fresh oregano and a squeeze of lemon juice, along with some salt and pepper to taste. Enjoy!