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 brown and no longer pink inside, about 7–8 minutes; transfer 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 outside, about 8–10 minutes; remove from heat and set aside.
Spoon one-third of the marinara sauce into the bottom of a baking dish to form an even layer.
Top the sauce with half the cooked chicken, then half the sautéed gnocchi, and half the sliced mozzarella.
Repeat the layers: another third of the marinara, the remaining chicken, remaining gnocchi, remaining marinara, and the remaining mozzarella. Sprinkle the grated Parmesan evenly on top.
Bake in the preheated oven until the cheese is melted and the casserole is heated through, about 10–12 minutes.
If desired, broil 1–2 minutes more until the cheese is golden brown—watch closely to prevent burning.
Remove from the oven, sprinkle with torn basil, then serve.