GtkGammaCurve a subclass of #GtkCurve for editing gamma curves. The #GtkGammaCurve widget is a subclass of #GtkCurve specifically for editing gamma curves, which are used in graphics applications such as the Gimp. The #GammaCurve widget shows a curve which the user can edit with the mouse just like a #GtkCurve widget. On the right of the curve it also displays 5 buttons, 3 of which change between the 3 curve modes (spline, linear and free), and the other 2 set the curve to a particular gamma value, or reset it to a straight line. The #GtkGammaCurve-struct struct contains private data only, and should be accessed using the functions below. Creates a new #GtkGammaCurve. @Returns: a new #GtkGammaCurve.