Cook the pasta in a large pot of salted water, until al dente. Drain and set aside.
Meanwhile, melt the butter in a large skillet over medium heat. Add the garlic, and cook for one minute. Whisk in the heavy cream, milk, Parmesan, onion salt, and pepper. Bring the contents of the skillet to a light simmer, stirring occasionally. Stir in the shrimp, scallops and crab. Continue to simmer until the shrimp and scallops are cooked through, stirring occasionally.
Pour the cooked pasta into the skillet, and stir well to combine. Stir in the Gouda cheese, and season with salt and pepper, to taste.