Aaahhh, I take it your a Yankee. Gas mark 5 is what we use in the UK for determining how high to set the oven when cooking stuff. Just to give you an idea, they go up to gas mark 9, and the lowest setting is 1 (dont know anything that cooks on that!). Beef on Sundays goes on gas mark 4/5, chicken (in a pack, the readymeal ones, like chicken Kievs and the like) go on around gas mark 6, and pizzas on gas mark 7. This should give you a rough idea of what temperature to cook at