Blizzard has announced that Defense Protocol Gamma will be on the PTR for testing soon, but it seems that the Defiler’s Scourgestone vendor has already been implemented. Defiler’s Scourgestone will be the new Sidereal Essence equivalent currency for Defense Protocol Gamma Dungeons.
As of right now, the vendor offers Trial of the Crusader 25-Player Normal Gear, with various prices that will likely change over time.