I am looking for the desendants of Jean Durrua who died 1692 in the Town of Castelsarrasin, Parish of Saint Sauveur, Tarn et Garonne, Region of Midi-Pyrenees, France. My oldest known ancestor is Peter Durrua born 1783 in Agen, Lot-et-Garonne and I am trying to make the connection.

This is a very interesting name. In the span of more than 150 years the name is found in France with relative scarcity. It does indeed come from Tarn and Garonne from the twon of Lavardac.
I suspect that the surname, DURRUA, probably came from the town of Duras also in Tarn et Garonne. Could it be that a lady of Duras was called a "Durrua", the feminine form of Duras? This will need some some further investigation into the spoken gascon of the area.

I confirm that Jean DURRUA who died in Castel sarrasin was from DURAS, a town 75 km away according to GASCON surname usage. Your Jean DURRUA's Family came most probably from DURAS..the problem is to find his Family real Surname. You may have an inkling as to what it could be if you look at the Family's first names, if you look at the work they did.
DURRUA is not the original name.
Place names were often used for new arrivals (immigrants) to new lands or new towns in Gascony.