Preheat the oven to 450°F (230°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; transfer the chicken to a plate and set aside.
If the skillet is dry, add another tablespoon of oil, then add the gnocchi and sauté until golden on the edges, about 8–10 minutes; remove from heat and set aside.
Spoon one-third of the marinara sauce into the bottom of a casserole dish and spread into an even layer.
Top with half of the cooked chicken, then half of the sautéed gnocchi, and half of the sliced mozzarella.
Repeat the layers: another one-third of the marinara, the remaining chicken, the remaining gnocchi, the remaining sauce, and the remaining mozzarella.
Sprinkle the grated Parmesan evenly over the top.
Bake in the preheated oven until the cheese is melted and bubbly, about 10–12 minutes; if desired, broil 1–2 minutes to brown the top, watching carefully to avoid burning.
Remove from the oven, sprinkle with torn basil, and serve.
