As stated in unfinished tales, Gandalf didn't know that Bilbo would find the ring on the adventure. He originally wanted to help Thorin since having dwarves in the lonely mountain would prevent Sauron from attacking Gondor and Lothlórien from the north. The ring finding it's way to Gollum and then Bilbo was almost definitely due to slight meddling from Eru (just as Gollum's death was due to Eru loosening the rocks under his feet) so Gandalf could orchestrate the fellowship's journey.