While the chicken marinates and before cooking begins, prepare all your fresh components. Thinly slice the red onion into half-moons, dice the plum tomatoes into bite-sized pieces, and shred the romaine lettuce. Chop the fresh parsley and set it aside. Having everything prepped and ready means you can assemble the gyros quickly while everything is still warm, which is key to a great eating experience.
Heat 1 tablespoon of olive oil in a large skillet or cast-iron pan over medium-high heat. Add your fries (fresh or frozen—no need to thaw) and cook until golden and crispy, stirring occasionally, about 8-12 minutes depending on thickness. In the final minute of cooking, sprinkle the garlic powder, dried oregano, smoked paprika, and salt and pepper to taste, tossing everything together so the seasonings coat evenly. I like to taste a fry and adjust the seasoning right before removing them from heat to ensure they're perfectly salted.
While the fries finish cooking, place a dry skillet or griddle over medium heat. Warm the pitas directly on the heat for about 30 seconds per side—this makes them more pliable and easier to fold without cracking. Transfer them to a clean kitchen towel to keep them warm and soft while you finish the other components.
Heat your grill or grill pan to medium-high heat. Cook the marinated chicken skewers until they're heated through and have nice char marks on the outside, about 8-10 minutes total, turning halfway through. Once cooked, let them rest for a minute, then carefully slide the chicken off the skewers onto a cutting board. You want the chicken warm and ready to use immediately when assembling the gyros.
Working with one pita at a time, lay it flat on a plate. Spread about 1/4 cup of tzatziki across the center of the pita, creating a base layer. Layer the warm seasoned fries from Step 2 down the middle, then arrange the grilled chicken pieces on top. Add the fresh lettuce, diced tomatoes, and sliced red onion. Sprinkle the crumbled feta cheese and fresh parsley over everything. I find that layering the feta and tomatoes last helps them stay visible and fresh-tasting rather than getting buried. Repeat with the second pita.
Carefully fold each pita in half, wrapping it around the filling like a sandwich. The warm pita should hold together naturally. If you need extra support, you can wrap the bottom half with parchment paper or foil. Serve immediately while everything is still warm and the fries are crispy.