Ressources

In German
For decades, the standard reference has been which went through several editions.
 * Werner Bätzing: Grande Traversata delle Alpi
 * Teil1: Der Norden. ISBN 3-85869-256-5. Rotpunktverlag Zürich,
 * Teil2: Der Süden. ISBN 3-85869-257-3. Rotpunktverlag Zürich,

Now another guide has appeared:
 * Iris Kürschner & Dieter Haas: GTA - Grande Traversata delle Alpi. ISBN 978-3-7633-4402-4. Bergverlag Rother München (2011).

For extending the GTA towards the Mediterranean:
 * Werner Bätzing/Michael Kleider: Die Ligurischen Alpen – Naturparkwandern zwischen Hochgebirge und Mittelmeer. ISBN 978-3-85869-432-4. Rotpunktverlag, Zürich (2011).

In English

 * Gillian Price: Through the Italian Alps: Grande Traversata Delle Alpi (GTA); Cicerone Press Limited, 2005; ISBN 1852844175, 9781852844172

In Italian

 * Eberhard Neubronner, Il Sentiero Grande Traversata delle Alpi GTA, dal monte Rosa al mar Mediterraneo, Zeisciu Centro Studi (no idea whether this is available and up to date)

Official web sites
Unfortunately, the "Assoziazione GTA" is inactive since long; it does not have a web site.

Of the concerned provinces, only Cuneo seems to be actively supporting the GTA:
 * http://montagna.provincia.cuneo.it/gta/percorsi/index.jsp?lang=it&percorso=GTA+PRINCIPALE Provincia di Cuneo

Part of the GTA coincides with the blue or red trail of the Via Alpina:
 * http://www.via-alpina.org/

Private web sites:

 * wanderweb.ch: official updates to the books by Bätzing and Kleider
 * http://www.wanderweb.ch/buecher/suedalpen-italien-pyrenaeen/westalpen-grande-traversata-teil-1/zum-buch.html Northern part
 * http://www.wanderweb.ch/buecher/suedalpen-italien-pyrenaeen/westalpen-grande-traversata-teil-2/zum-buch.html Southern part
 * http://www.wanderweb.ch/buecher/suedalpen-italien-pyrenaeen/ligurische-alpen/zum-buch.html Ligurian Alps
 * http://www.klingenfuss.org/gta_wege.htm leg-by-leg information on the web site of Jörg Klingenfuss
 * http://www.facebook.com/gta.trek photos by guide authors Iris Kürschner & Dieter Haas

Ultralight equipment

 * http://www.randonner-leger.org French MUL (marche ultra-legère) forum