Heat beer in a saucepan set over medium heat until it just starts to simmer.
Into a bowl, toss together cheddar and gruyère with flour until coated. Add the cheese to the beer, and stir until it starts to melt. Add pumpkin, worcestershire sauce, salt, mustard, and nutmeg. Stir until the cheese is completely melted and the dip is smooth.
Serve warm in a hollowed-out pumpkin with warmed sliced bratwurst or pretzel bread.