What can I substitute for Gruyère cheese? I haven't a clue what Gruyère cheese is or what I could sub for same taste in Cheesy Biscuit Casserole.

Gruyère cheese is a light yellow firm/hard cow's milk cheese that's mellow, buttery and nutty.  It melts well and is often used in baked dishes (like the Cheesy Biscuit Casserole), cheese fondues and onion soup.  It's also great in salads and on a cheese tray.  It's named after a city in Switzerland.  And, yes, Swiss cheese would be a great substitute, like in this Gruyère Vegetable Bake, since it has similar characteristics depending on the age.  Mmm, warm melted cheese sounds pretty tasty right now!