Add the garlic cloves to the bowl of a food processor fitted with the blade attachment. Pulse until very finely chopped.
Add the quartered onion and pulse again until smooth.
Add the meats & spices to the food processor and pulse until the mixture becomes a paste- about 1 minute. Scrape down the sides of the bowl with a spatula every couple pulses.
Transfer the mixture to a 9x5" metal loaf pan. Using a spatula, or a clean hand, press down and compact the mixture into the loaf pan so that it fills the entire pan and no air pockets are left.
Bake the gyro meat loaf at 350° for 45 minutes - 1 hours, until cooked through and the internal temperature reads 165°F.
Carefully remove the hot pan from the oven, and let is rest until the meat's cooled completely to room temperature.
Transfer the pan to the refrigerator and chill for, at least, 2 hours.
Remove the pan from the fridge, and run a knife around the edges of the loaf to release it from the pan- then transfer the meat loaf to a clean cutting board.
Using a serrated knife thinly slice the meat. Pour all of the juices from the dish evenly out over the meat.
You can now freeze the meat or continue on to fry it and make homemade gyros.