Issues with coordinate system.
Most engineering 3D software use Cartesian coordinate system (y-axis -ve starboard +ve port) as default, however in Naval Architecture it is more likely to use Transverse (y-axis) +ve starboard -ve port.
When importing / exporting gdf/stl/obj files to / from MAXSURF Modeler the model can invert, because of the difference in coordinate system.
To avoid confusion and getting model inverted, try to change coordinate system in your 3D software to match MAXSURF Modeler's axis directions.
Please note when importing IGES and DXF files it is possible to select direction of each axis.