Very simple example to build a GUI :

   win := BLOX.BWindow new: 'Example' .
   c := BLOX.BContainer new: win .
   c setVerticalLayout: true.
   c defaultHeight: 100 .
   c defaultWidth: 100 .
   l := BLOX.BLabel new: c label: 'WAITING'. 
   b := BLOX.BButton new: c label: 'Click me'. 
   b callback: [  l label: 'Hello World' ] message: #value.
   win map .
   BLOX.Blox dispatchEvents: win . 
   'Finished' printNl . 

The GNU Smalltalk GTK Tutorial : http://bioskop.fr/gtk_tutorial/