that's so funny i had no idea gareth could attack you
He attacks you if you got Fane in your party. Not shapeshifted, that is.
I have Fane as my "main" character (not shape-shifted) and Gareth did not attack me through all of Act 1 and a few hours into Act 2. Do you think Fane has to be a companion for Gareth to attack him? Maybe there is something else causing him to be hostile (villain tag?).