DesertHawk has explained the system fairly well. The reason why "nobody knows" for the Boeing 767 is that until recently, people on the Boeing 767 would generally remain on the Boeing until their upgrade came. There were a few transitions to the Airbus fleet, but few. The future on that aircraft is the million dinar question, because when André Dosé came on-board as the CEO, he slated the Boeing 767's for retirement within a relatively short time frame. Mr. Dosé is gone, but the fact of the matter is the 767's are at the point where they will need to be replaced, so nobody really knows what the future holds (or at least those who know aren't talking, and those who are talking don't know).
At this point in time, Gulf Air is the only airline in the region that (thankfully) does not have a Direct-Entry Captain scheme, and the airline seems to have committed itself to clarifying seniority and honoring a seniority-based career progression system. There are sins that have been committed in the past and some guys have some legitimate grievances, but at least as of this week, the Company seems to be making the right moves towards going straight.