Preheat the oven to 450°F (232°C).
Heat 1 tablespoon olive oil in a large skillet over medium-high heat.
Add the cubed chicken and cook, stirring occasionally, until golden and cooked through, about 7–8 minutes; remove chicken from the pan and set aside.
If the skillet is dry, add another tablespoon of oil, then add the gnocchi and sauté until golden on the outside, about 8–10 minutes; remove and set aside.
Spoon one-third of the marinara into the bottom of an oven-safe baking dish and spread it into an even layer.
Top the sauce with half of the cooked chicken, then half of the sautéed gnocchi, and half of the sliced mozzarella.
Repeat the layers: spoon half of the remaining marinara over the cheese, add the remaining chicken, remaining gnocchi, then the rest of the marinara and remaining mozzarella. Sprinkle the grated Parmesan evenly over the top.
Bake in the preheated oven for 10–12 minutes, until the cheese is melted and bubbly. Optionally broil 1–2 minutes to brown the cheese on top—watch closely to avoid burning.
Remove from the oven, top with torn basil, and serve.