gtk_preview on SGI O2

[abaldoni CsR UniBo IT (Alessandro Baldoni mat.1010)]

I'm experiencing a lot of problems using the gtk_preview widget on a
Silicon Graphics O2.
Simply, whenever I use it, it gives me a Bad Match X11 error.
The same application on my Linux box works smoothly.
I'm developing an extension for the GIMP.
Also note that testgtk gives me the same error with the preview_color
and preview_gray tests.
I tried to use
  gtk_preview_set_gamma (gimp_gamma ());
  gtk_preview_set_install_cmap (gimp_install_cmap ());
  color_cube = gimp_color_cube ();
  gtk_preview_set_color_cube (color_cube[0], color_cube[1],
  color_cube[2], color_cube[3]);
  gtk_widget_set_default_visual (gtk_preview_get_visual ());
  gtk_widget_set_default_colormap (gtk_preview_get_cmap ());

as in all the plug-ins, but it doesn't work.

Thanx.