Alex wrote on twitter that it's "family matters". I hope all is fine with Greg and his loved ones. All the best to him!
I wonder how JG will manage to learn all the songs on such a short notice. Too bad SDG wasn't available.
Anyway, hopefully Greg will be back in the game ASAP.
Edit: of course, people on BBM are speculating about drugs. Hopefully Greg doesn't give a shit about gossip. Hang in there man, Testament needs you!