You insert Gap keys to edit optical data between two segments.
This button is only available when the Gap Interpolation is set to Control Curve.
To insert gap keys within a gap, select Control Curve in the Gap Interpolation menu. Move the Timeline indicator to the location within the gap where you want to add a key and click Insert Gap Key. This added key does not modify the curves themselves, it glues the control curve to the original curve. You can then modify these keys and change the original curve. Double-click the added key to select the gap key. When selected, you can drag the key to change its value or you can use its handles to adjust the slope of the control curve.