Bring a large pot of lightly salted water to a boil. Add penne and cook, stirring occasionally, until tender yet firm to the bite, about 11 minutes. Drain pasta and return to the pot.

Combine flour and oil in the same pot and mix until a smooth paste is formed. Add oat milk slowly. Incorporate pumpkin, whisking until thoroughly combined. Add garlic salt and 4 ounces vegan cheese. Mix until smooth.

Set an oven rack about 6 inches from the heat source and preheat the oven's broiler.

Fill 3 individual oval gratin casserole dishes with the pasta mixture. Top equally with remaining vegan cheese.

Broil in the preheated oven until cheese is melted, 1 to 3 minutes. Serve warm.