I don't believe there is any agreement between the companies. There are several Gulf Air pilots at Etihad. If your company will give you a "No objection" letter, basically saying that your present company has no problem with you quiting them to go work for the competition, then you can apply and join right away. If not, then you have to quit your job and leave the Gulf for six months. Nothing to do with the "Prestigious" Etihad but with the UAE labour law.
And to the rest of the Etihad bunch, lets stop airing our dirty laundry here for a while and give these new guys the chance they deserve.