Combine the onion, garlic, spices, herbs and bread crumbs in a large bowl. Mix everything well. You want to add the ground meat last to make sure you do not overwork the meat.
Mix the meat in with all the other ingredients until just combined.
Roll the meat mixture into golfball size meatballs. You should be able to make 18-24 meatballs, depending on how big you roll them.
Place the meatballs on a foil lined baking sheet and bake for 16-20 minutes. If you have 24 meatballs they should only take about 16 minutes, if you made less they will take a little longer to cook.
Take the meatballs out of the oven and let rest.
For the tomato cucumber salsa: Combine all the ingredients in a bowl. Season with salt and pepper. You can make this ahead of time and store in the fridge up to 24 hours prior.
Heat the pita bread in the microwave or oven.
Build the gyros: Liberally coat the middle of the warm pita with tzatziki sauce. Place 3-4 meatballs down the center of the pita, on top of the sauce. Top the meatballs with the tomato cucumber salsa. You can garnish everything with feta and fresh herbs, but that is totally optional!