Import the .gmt in 3DSimEd (Not "Import as Object", but the one that just says "Import"), save it and a window should then appear with the pivot point coordinates. Unless it's set to 0.0 0.0 0.0, it won't rotate properly. It's the same principle for those cars that have misplaced wheels in showroom (usually all condensed at the center of the car) for example.
In poor words, pivot point for all parts must be set to 0.0 0.0 0.0 in 3DSimEd. The only exceptions with "custom" pivot points are the steeringwheel and the helmet for the driver.