Gandalf sounded like he suspected that something was amiss about the Ring even when he discovered its existence during the timeline of the Hobbit. It seems odd that he NEVER consulted Saruman, Radaghast, or the Blue Wizards about it. It's not like Gandalf told NOBODY about it either, as I suspect he tipped off Aragorn at any rate. As there was, as far as we know, not enough warning to make Gandalf believe that Saruman had turned traitor, why did Gandalf keep the Ring a secret? For all he knew, not asking Saruman and the others for help could have delayed him long enough for Sauron to win the War of the Ring.