The day before you plan to cook these, mix up the marinade ingredients (excluding chicken) in a bowl (use one that will fit all your chicken inside). Whisk till oil is well incorporated. Taste and add salt as needed. When seasoning is correct, add chicken and cover. Refrigerate overnight. If you're in a hurry (i.e. you're not doing this the night before), let the mixture marinate on the counter for at least 30 minutes before cooking.
Also the day before you plan to cook these, make the sauce. Place yogurt in the center of a clean piece of cheesecloth (I use a kitchen towel--it works just as well). Bring up the sides and twist so that yogurt is all wrapped up in a nice little bundle. Secure towel and hang yogurt over your sink (I do this at night and just tie the towel around the arm of my faucet). Let yogurt drain overnight (12-24 hours). The longer it drains, the thicker your sauce will be. When yogurt has drained to your liking, scrape it off the towel/cheesecloth and into a small bowl. Add lemon juice, diced cuke, and garlic; mix well. Taste and adjust seasoning with salt if needed (some yogurt brands are saltier than others, so you may not need any salt at all). Let sit for at least 15 minutes before serving so flavors can blend. This gets better and better the longer it sits! (Again, if you're in a hurry and not doing this the night before, just stir everything together for the sauce--it'll be runnier, but you'll live!).
When you're ready to make the gyros, preheat your grill (charcoal is best); meanwhile, prep your tomatoes and onion. Thread chicken onto soaked skewers, leaving a little space between each piece. Cook skewers for a couple minutes on each side, turning once, till done.
When chicken is all cooked, throw your pitas on the grill to heat up (and get that smoky/grillly taste!), turning once. When your pitas are hot (and nice and flexible), take them of the grill. Working with one pita at a time, place pita on foil in your hand. Lay a skewer down the middle of the pita, and curl the pita around the meat with your hand while you pull out the skewer. Open the pita back up and top with tomato wedges and onion, dollop with the sauce, then wrap foil around the bottom half of the pita securely (so it won't leak and/or lose any of the filling!). Eat!
These are great fresh, but I usually cook extra to have for other meals during the week--keep all the ingredients separate till you're ready to eat, then just reheat the chicken and pitas in the microwave before assembling. I also often take these as camping food--freeze the chicken in the marinade and let it marinate as it thaws (usually takes 2-3 hours)--cooks the same from that point (and no messy dishes to wash!).