The Past Kages would win. Gaara's Sand is a great counter to the Clam, but even Gaara's Sand took a good deal of time finding the Calm, which allowed Mizukage to attack him several times before than. So the Past Kages would get off several free attacks against the Gokage, which all they could really do is rely on Gaara's Sand to block, but most of the Past Kages attacks will just plow through that sort of defense, especially if weakened by Mizukage's Oil. So I expect the past Kages would just kill Gaara before he could find the clam. Than w/ no one else who can find the Clam, the Past Gokage proceed to rape the remaining Gokage under the cover of the Mist.
Only way this could be avoided is if Onoki used his massive Cube Jinton to just nuke the whole battlefield, but it's unlikely that he pulls that out so early into the match and even that is not a guaranteed win.