A brilliant question, really. I have often wondered about Gandalf's motivations in this as well. I do not believe he had any inkling that the Ring would be found. However, in my mind, his intention was to re-establish dwarven control of the Lonely Mountain, thereby providing allies to the north of Mordor. In any case, he undoubtedly had a knowledge that this action would result in some good for the West in its fight against the Darkness.