Preheat broiler.
Cook gnocchi in a 5-quart pot of boiling salted water according to package instructions (gnocchi will float to surface when done). Drain in a colander.
Whisk together cream, flour, salt, pepper, and nutmeg in a 12-inch ovenproof skillet, then bring to a boil over moderate heat, whisking. Continue to boil, whisking frequently, until reduced by half, about 2 minutes. Add spinach in handfuls, tossing with tongs, and cook until wilted, 2 to 4 minutes. Remove from heat, then stir in gnocchi. Spoon ricotta over gnocchi in 5 large dollops and sprinkle with mozzarella. Broil 4 to 6 inches from heat until cheese is browned and bubbling in spots, about 2 minutes. Season with pepper.
