I have the gt680 and seldom toutch it but seem to remember you can't access the menu while its recording.. I have the g sensor off as being in a cabrio I don't want locked files every time I put the roof down !
On the right side there are two buttons, top right side is "M". When during recording you press it, - it will lock file.
In order to go into setting menu, you need to stop recording ( OK button on top right edge ), then press "M" on right top side.
If you want to playback videos, first need to stop recording ,then click "Mode" button on the right bottom side of the unit.