gdk:window-set-title (window title)

window -- a toplevel gdk:window object
title -- a string with the title of window

Sets the title of a toplevel window, to be displayed in the titlebar. If you have not explicitly set the icon name for the window, using the gdk:window-set-icon-name function, the icon name will be set to title as well. title must be in UTF-8 encoding, as with all user-readable strings in GDK/GTK. title may not be nil.