Tim,

You need to do a read up on what is HH gyro and understand what it does. Why must you assume the servo arms needs to always return to 90 degree? I don't get the assumption you are having. In HH mode, the gyro's servo doesn't return the servo arm to 90 degree but this is true only in rate mode.

Since you have change the setting then this is what you need to do to restore it.

Connect the servo directly into ch 4 and bypass the gyro and set the rudder trim to center. Then reposition the arm and make sure it's 90 degree to the servo case. The move the servo on the boom so that the pitch slider is in the center of travel on the tail rotor shaft. lock the servo tight on the boom and then connect the servo back to the gyro.

The switch on left should be normal and the switch on the right is REV. Make sure your tail servo rod is a straight shot.

Andy