In a large sauce pot (could be the same used for pasta) melt 4 tablespoons of butter over medium heat. Whisk in the flour followed by the garlic and lemon zest. Cook for another minute, then add the half and half and whisk until smooth.
Once the mixture is warm, add the cheeses. Stir to blend, then add the salt, pepper and nutmeg. Once smooth, taste for salt (add more if needed) and pour the cheese mixture over the crab and gnocchi. Mix and smooth in the baking dish.
Then melt the remaining 3 tablespoons of butter and toss with the bread crumbs. Sprinkle the top with bread crumbs, and bake for 20-30 minutes, until the edges are bubbly and the top is golden.