Hello:
A few questions are in order to help us help you:
Is this for MS-FSX
or Prepar3d (aka "P3D") ?
Did you export the model from Sketchup, then import it into ModelConverterX (aka "MCX") ?
If so, when the model was imported into MCX, did you use the Ground Polygon Wizard to make the "G-Poly" ?
If you used the MCX Ground Polygon Wizard to make the "G-Poly", did you assign the correct attribute for MS-FSX versus P3D, which determines whether to use a VTP layer number (FSX)
or a Z-Bias value (P3D) ...to slightly offset the rendering of the "G-Poly" relative to the ground plane of the Airport Background ?
Did you also use a "flatten" polygon attribute when you made the Airport Background polygon ?
If so, in the "flatten" polygon attribute (when you made the Airport Background polygon), did you set the Altitude / elevation of the flatten at the
same value in Feet or Meters ...as the Airport Reference Point (aka "ARP") and Runways ?
Hope this helps sort out the FSX or P3D run time "G-Poly" visual anomalies you are showing in the screenies above.
GaryGB