It is possible (albeit a bit complicated) to go the other way and create a GdkPixbuf image from a GdkDrawable. This can be used to take a pixmap that had something drawn on with GDK operations and then scale it or composite over another image, or capture contents of an arbitrary window. 

<?php
 GdkPixbuf gdkpixbuf::get_from_drawable(GdkPixbuf   $dest,
                                        GdkDrawable $src,
                                        GdkColormap $colormap,
                                        int         $src_x,
                                        int         $src_y,
                                        int         $dest_x,
                                        int         $dest_y,
                                        int         $width,
                                        int         $height)
?>
Note that this function is not a GdkPixbuf method, since $dest can be a null. If $dest is null, the function will create a GdkPixbuf of the dimensions given by $width and $height and will transfer the graphical contents of the specified drawable to the pixbuf. In this case, $dest_x and $dest_y have to be 0. If $dest is not null, then the function will transfer the pixmap at the destination coordinates.