Gamma correction using GDK/directly accessing pixmap pixels

[Hongl Lai <hongli telekabel nl>]

I am writing a 2D game using GTK and GDK, and I need to implement gamma
correction to do things such as a screen that goes black slowly.
Is it possible directly access a GdkPixmap's pixels?
GdkRGB is not an option, because I can't use pixmaps.
Or is there any other way to do gamma correction using GDK?

There is some code in gnome-core (the panel) that does something
similar.
I have modified that function for gamma correction.
However, since it use GdkPixbuf, it's waaay too slow because I need to
convert a GdkPixmap to GdkPixbuf, do gamma correction, and then render
it to a GdkWindow.