The GTA stands for the Grande Traversata delle Alpi, a long distance hike through the Italian Alps.
A good English language guide is published by Cierone Press: Through the Itralian Alps, GTA by Gillian Price.
The total distance of the south to north path in the Cicerone Guide is 633 kilometers, with 44,000 meters of ascent and 44,000 meters of descent over 47 stage-days.
The Italian Alps are actually five different mountain chains, from south to north:
Ligurian Alps: from the Mediterranean to Colle di Tenda, mostly limestone
Maritime Alps: from Colle di Tenda to Valle Stura, contains igneous and metamorphic gneiss and granite and glacial molding. Parco Naturale delle Alpi Maritime.
Cottian Alps: from Valle Stura to the Valle di Susa, includes 3841 m Monviso. Limestone and metamorphic greenstone.
Graian Alps: from the Valle di Susa to the Valle d'Aosta. Metamorphic rock with extensive glaciation. Includes Mt Blanc and Gran Paradiso. Parco Nazionale del Gran Paradiso.
Pennine Alps: from Valle d'Aosta almost to Domodossola, one of the main passes into Switzerland. These central Alps include 4634 m Monte Rosa, Europe's second highest mountain, and the Matterhorn.
We hiked for 13 days, covered 196 km; climbed more than 13,000 m and descended the same amount.