Bring a large pot of salted water to a boil. Cook the macaroni two minutes less than what the package directions dictate. Drain and reserve.
Melt butter in the pot over medium heat. Whisk in flour. Continue to whisk constantly for two to three minutes. In the last 30 seconds, add garlic.
Pour in milk and heavy cream, whisking constantly to create a smooth mixture. Stir in Dijon mustard, salt, pepper and nutmeg. Bring to a simmer.
Turn off the heat. Stir in Gruyere and cheddar until melted, and then stir in the pumpkin purée until it becomes one cohesive mixture.
Once the cheese fully melts to make one cohesive mixture, added the reserved macaroni. Stir to coat the pasta.
Pour the macaroni into a baking dish and top with crushed Ritz crackers and melted butter. Bake until hot and bubbly.