Doubling a gateau cake recipe?

Longer cooking time is the main thing. You also don't need to double the baking powder and soda if the recipe calls for them. About one and a half times should be enough. If the pan is big enough it should cook through to the center without a problem, just in a longer time. But it shouldn't need twice as long. If the recipe calls for an hour, I would start checking it at that time, and then check it again about every 10 minutes.