Hi @chengjh1 and thanks for your query. The gaze co-ordinates as reported by the Core SDK are reported as normalised in reference to the screen area itself, namely the values of x,y lie between 0,1 therefore values outside the screen area are not necessarily viable for use.
Did you mean to suggest that when you turn your head more than 30 degrees, the eyes are no longer captured? So long as the eyes are looking within the screen area, then trackability should still be OK.
It would be helpful if you could kindly describe in more detail the scenario you are working with and we will do our best to help you out!