Cook garlic in butter in a large saucepan over medium heat until fragrant, 1 minute. Sprinkle with flour; cook, whisking, 1 minute. Slowly whisk in half-and-half. Simmer, whisking often, until thickened, 6 to 7 minutes. Remove from heat and whisk in 6 ounces (about 1 1/4 cups) Gruyère; fold in Brussels sprouts. Season with salt and pepper.