On the gateway page, "type 2D" means that there is no building described in this package.
On the org, you should have buildings. Make sure you followed the installation instructions correctly. They are probably included in the downloaded archive (note that you need to unzip it).
Most of the time, no building is displayed when there is a missing library. You can find clues in the .../X-Plane 11/log.txt file, after having tried to load a flight at the malfunctioning airport.
It can also be that you placed one extra layer of folder (check the hierarchy of ...X-Plane 11/Custom scenery/aerosoft folders for comparison).
This forum is mainly meant for X-plane default install, so for more help, you should check X-pane.org forum or contact the artist who created GMMX.
All the best with that,