Cook the gnocchi according to package directions. Drain and set aside.
Into a medium-size saucepan over medium heat, add the butter. When the butter is melted, add the garlic and cook for about 30 seconds, stirring frequently. Whisk in the flour. The mixture will be thick.
Gradually add the oat milk, whisking continuously for 2 minutes, until the sauce thickens.
Whisk in the cheddar cheese, salt, and ground black pepper. Cook for 3 minutes, stirring frequently until the cheese is melted.
Remove from the heat and stir in the sour cream.
Off heat, return the gnocchi its cooking pot. Pour the cheese sauce over the cooked gnocchi, stirring gently to coat. Serve hot.