In a large bowl mix together the lemon juice, plain Greek yogurt, salt, oregano, and paprika. Add the chicken tenders to the marinade and toss to coat. Cover the chicken tenders and transfer the bowl to the refrigerator to marinate for at least 30 minutes to overnight.
To prepare the tzatziki sauce, grate the cucumber using a cheese grater and squeeze as much liquid from the cucumber as possible. Transfer the cucumber to a medium bowl and combine with the Greek yogurt, garlic, red wine vinegar, fresh dill, salt, and pepper, to taste. Cover and transfer the tzatziki to the refrigerator until ready to use (keeps well for approximately 2-3 days).
Heat the olive oil in a large skillet over medium-high heat. Once hot, add the chicken tenders to the skillet (working in batches if necessary). Cook the chicken tenders for 5 minutes, undisturbed, before flipping to the other side to cook for an additional 5-10 minutes more, or until fully cooked.
As the chicken cooks, warm the pita bread in the microwave or oven and prepare the desired toppings.
Build each gyro by spreading Tzatziki onto each pita bread, and top with fresh sliced tomatoes, red onion, and 2-3 chicken tenders. Wrap up pitas in parchment paper or foil and enjoy!