I would say, first slew all over the airport to see for yourself that a ground surface can be created for you by a 1 Meter terrain mesh.https://www.fsdeveloper.com/forum/threads/sloped-flatten-blend-into-ftx-eu-england-mesh.445544/post-825225
Nice , so how do I proceed with realistically creating SDGCs terrain?
All you may need to do now, IMHO, is focus on making terrain textures to be draped- or 3D models to be placed- onto / into the terrain mesh.
FYI: Elevation data and ground shape in the final 1 Meter DTM terrain mesh is likely to be identical to elevation data and ground shape of all those "Alpha build" demos in the areas where you place 3D scenery objects, but it shall be blended with no spikes / pits / cliffs etc.
An exception, of course, would be areas where you wish to modify the terrain mesh by using a CVX vector sloped / level flatten BGL for RWYs, Taxiways, Aprons etc. that the SDK requires to be "level" where AI traffic / Ground Vehicles will be displayed (if any are to be used).
In SBuilderX you can assign a "Constant" Altitude to all vertices of a selected flatten Polygon to make it a "level" surface in FS at run time:
See: SBuilderX Help File topic...
"Working with Points, Lines and Polygons
Vector Lines always cling to the altitude mesh and you cannot modify their height. But, you can set the altitude of textured Lines, of Lines of Objects, and of certain Polygons (Airport Flattens and Hydro Polygons). Right-click on the Line segment or on the edge of the Polygon and choose Set Altitude from the menu - make your changes in the Altitude dialogue:
In the case of a Hydro Polygon, you can also determine the slope."
NOTE: Both Hydro Polygons and Airport Boundary Polygons require an Altitude assigned to them, or they will default to 0 Meters AMSL.
Hope this helps.