I had the same problem of no Gareth at the rally point in my game, in version alpha v2.0.165.341. In my case I was playing by myself, no online/coop. I think it's a bug and no screwup by you, unless he is standing around near the rally point someplace we both missed. WARNING: POSSIBLE SPOILERS FOLLOW.
It sounds like others don't have this bug and Gareth gives info and possible help for the last battle. However you can still fight the last battle and win without him, at least in my game. After following the Seekers to the boat they take, head for the courtyard of the castle. A tough encounter should start, and if you win you can get onto another boat.