Prepare the aioli sauce by mixing mayonnaise, garlic and lemon in a bowl; season with salt and pepper to taste, transfer to a dish, press plastic wrap on surface, and refrigerate while lamb is cooking.
Preheat grill to medium-high. Season lamb with salt and pepper, and grill to desired doneness, about 4 minutes per side for medium-rare.
In the meantime, in a large pan melt the butter over medium-low heat. Add the gnocchi and sauté for 5 minutes. Serve the grilled lamb on a plate, adding the aioli sauce on the side and the gnocchi. Garnish with sprigs of rosemary.