Preheat the oven to 375 degrees . Bring a large pot of salted water to a boil. Stir in the spinach and cook until just wilted, about 30 seconds. Using a slotted spoon, transfer the spinach to a large colander, pressing out as much liquid as possible. Transfer to a 9-x-13-inch baking dish.
Return the water to a boil. Add the gnocchi and cook according to package directions; drain and transfer to the baking dish. Add the tuna and pesto and stir to combine; season with salt and pepper.
In a food processor, pulse the ciabatta to fine crumbs. 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