I understand the visible handle of the gust lock was observed to be in the OFF position, and that portion of the cockpit appears intact.
It's my understanding that if the gust lock feature was engaged he couldn't have advanced the throttle for TO
That is what baffled me at first until this clue.
On the legacy Gulfstream models, G550 and earlier, the control column will fall full forward (nose down) when the control lock is released and the controls are not air loaded.
The actual locking device is armed to engage or disengage with the lever along with the throttle block.
However, in some form or fashion mechanically or hydraulically the elevator mechanism has to be separated from the yoke and commanded down for the lock to engage.
So obviously the falling forward of the yoke realigns the yoke or doing control checks accomplish the same result of resetting the yoke to the elevator commands.
If the pilots were holding neutral position of the yoke, released the gust lock on taxi and then the lock itself would disengage, BUT did not do control check then the yoke would not reset itself to align with the elevator control commands.
So by falling out of normal start sequence they missed both opportunities to realign/reset the elevator functions.
That is my limited interpretation anyway.