Turning the proportional all the way down is not bogus info. The proportional setting of the 3 in 1 is the same as revo mixing in a radio, it will add rudder commands as power increases to offset the increased torque. In HH mode the gyro will see these rudder commands as heading changes and will drift. I've got the proportional pot turned all the way down and my G90 holds just fine. It is normal to have to adjust the rudder trim to stop drifting as stated in the G90 manual.

If the gyro is having trouble holding the tail it's also possible your tail motor is on the way out.