It's very unlikely that one of the GB members realises it's all a charade. The reason why I believe this is that watchtower career progression path (e.g. MS->Elder->Bethel->Bethel Elder->CO->DO->Branch Overseer->GB) acts as a kind of filter; in that, anyone who could possibly have it in them to realise that it's all a load of baloney, would have awakened by the time they got to GB level.
On the other hand, if one of the GB members was to wake up all of a sudden. It might be possible that they would feel trapped and continue to maintain the charade for fear of coming out and questioning the organization/system which looks after them so well.
It makes me wonder: If I were a GB member and I woke up all of a sudden, realizing it was all a charade, what would my exit strategy be?