They used both light and dark Force powers for the good of the Universe.
Details not known.
The Order sought out similar orders to form alliances with. They made contact with the Night Jedi, who sent Jobie Wan Kenobi to act as ambassador. They agreed to an alliance, with students from either order taking part in exchanges with the other order, and military support if it was asked for. However, the alliance was not often used, and communications between the orders dropped off. The Grey Sith didn't form an alliance with the Light Sith because the Light Sith were wiped out.
The GSO were attacked on Ossus, and after a battle, were forced to retreat. They contacted their allies the Night Jedi, who offered them sanctuary on Endor, but they were not heard from again.