The flights are assessed one by one. On Friday, the captain advised of the possibility when we were in the lounge. He came through to use the PA.
During the flight he gave updates as to how the fuel burn was going. For example, external air temp was 10 degrees C higher than typical for the time of year and that persisted to more than the half way mark. We only had confirmation that we were going direct to NYC at the diversion point, just over two hours in. We made NYC in 3hrs 24mins.
Gander is chosen, I understand, since on departure the a/c can once again go supersonic on the 'hop' down to NYC and so make up some time.