gtk2/docs/reference/gdk/gdk-overrides.txt

# GdkPixmap, GdkBitmap and GdkDrawable are the same as GdkWindow.
<STRUCT>
<NAME>GdkPixmap</NAME>
struct GdkPixmap
{
  gpointer user_data;
};
</STRUCT>
<STRUCT>
<NAME>GdkBitmap</NAME>
struct GdkBitmap
{
  gpointer user_data;
};
</STRUCT>
<STRUCT>
<NAME>GdkDrawable</NAME>
struct GdkDrawable
{
  gpointer user_data;
};
</STRUCT>