It says in the last chronicle that the Jedi on Katarr were killed by Sith assassins, and the Sith knew of the gathering because Atris leaked the information. In the game, it's stated that everyone was wiped out by Nihilus' force drain thing, and he was drawn to the planet because of his hunger for the force. I realize the last part isn't necessarily contradictory; but it's heavily implied that he felt the gathering through the force, rather than coming across information that it was being held.