Soak dried mushrooms in enough boiling water to cover about 15 minutes or until soft. Drain, discarding liquid. Squeeze mushrooms to remove additional liquid.
Meanwhile, for mushroom sauce:
In a 4- to 5-quart Dutch oven, cook and stir leek and garlic in hot butter over medium heat for 2 minutes.
Using a slotted spoon, remove leek mixture.
Add oil to Dutch oven; heat over medium-high heat. Add porcini mushrooms & button mushrooms, and cremini mushrooms. Cook about 15 minutes or until mushrooms are lightly browned and liquid is evaporated, stirring occasionally.
Stir in wine, salt, and pepper.
In a small bowl, whisk together cream and flour; stir into mushroom mixture. Cook and stir until thickened. Cook and stir for 1 minute more. Stir in leek mixture and parsley.
Meanwhile, cook gnocchi according to package directions. Add gnocchi to the mushroom sauce. Serve gnocchi with mushroom sauce.