After nearly six months in the breakfast berth on ABC’s “Good Morning America” — in a move that the network called only temporary — the show’s cohosts Charles Gibson and Diane Sawyer don’t look to be leaving anytime soon. In fact, they will remain in place until at least May of 2000. “My wife keeps asking me how much longer we’re going to do this,” Gibson told TV writers at a West Coast news conference, “and I don’t have an answer for her.” The “GMA” ratings free-fall has been halted with the installation of Sawyer and Gibson (who helped toughen up the show’s soft image), though “America” still lags in numbers behind NBC’s “Today” show.