Neither do I, it was apparent that DK never did any research on Gravus or he wouldn't have done that to Gravus. In my world Gravus is still alive.
I guess we could assume it's a different Gravus - the game mentions there being many Darth Jugoran's. Unless the book specified that this is the Gravus that destroyed Taris, in which case it's just another example of DK butchering a character to help his own "pet" characters. (coughMeetracough)