In order to fool Iwan unto beleiving his on his side, Gajeel may have passed information on Makarov's order. So Iwan revealing he got it from Gajeel would earn distrust from the Fairy Tail members. He's just starting to win over some of them after what he did... this could hurt his chances for a while until he redeems himself. A point was made about only Makarov knowing this... so it would be foolish of the author not exploring this whenever Iwan makes his move.

Unless Makarov told the S-rank Mages (Erza, Gildartz and Mirajane) about his plans... everybody will believe Gajeel to be a traitor. And I don't see them looking well on traitors.