Ghaston Grey is a Dornish prison located on a bleak island in the Sea of Dorne. It been used by House Martell as a prison for the vilest of criminals and traitors, who are sent there to waste away.
Ghaston Grey is an old, crumbling castle on a rock. It is considered a horrible place to be sent.
A Feast For Crows
Following Princess Arianne Martell's failed attempt to name Princess Myrcella Baratheon as Queen of Westeros, her co-conspirators are sent to Ghaston Grey to await their sentences from Doran Martell, Prince of Dorne. Sylva Santagar is ordered by her father, Ser Symon Santagar, to wed Lord Eldon Estermont, and Garin and Andrey Dalt are eventually sent by Doran to serve for a time in Tyrosh and Norvos, respectively.