Without a car 'Cap de la Carrere' is a little difficult to get to, and you would miss out on much of what the area has to offer.
However, if you fly into Toulouse it is possible to get Ganties: By airport bus from the airport to the train station: Then by train direct to Saint-Gaudens: A local taxi to Ganties.
This is a good way to travel if, say, a member of you party is joining you later.