A copy of this tutorial in SGML and HTML is distributed with each source code release of GTK+. For binary distributions, please check with your vendor.
A copy is available online for reference at http://www.gtk.org/documentation.html#Tutorials. You can also find other GTK+ tutorials there.
A packaged version of this tutorial is available from ftp://ftp.gtk.org/pub/gtk/tutorial which contains the tutorial in various different formats. This package is primary for those people wanting to have the tutorial available for offline reference and for printing. Note that the packaged version may be older than the online version.