The Gilbert engines have a single field wired in series with the armature. If the engine runs in one direction but not the other it is the tender mounted reverse unit. Likely one of the copper fingers used in the reverse position is out of adjustment. The other possibility is the new 4 wire harness was connected wrong. The 5th wire only powers the light and smoke unit, it is not used for the motor.
It is tempting to call a Gilbert reverse unit (also referred to as a remote control unit) an E unit but it is not the same. The Lionel E unit is for motors with double wound fields. Gilbert used motors with double wound fields in some accessories but not in engines.
