Preheat the oven to 425 degrees. In a medium saucepot, bring water to a rolling boil. Add the gnocchi to the pot and cook for 3-4 minutes, or until they float to the surface. Add spinach in at the last minute of cooking to wilt. Drain and transfer to the baking dish. Add the tuna and pesto, stirring to combine. Add salt and pepper to taste.
In a medium bowl, toss the breadcrumbs and cheese. Sprinkle the breadcrumb mixture evenly over the gnocchi and bake until heated through and golden, about 15 minutes.
Serve warm with extra pesto or olive oil drizzled on top if desired.