public SpatialReference Gcs {get;}

// Use a builder convenience method or use a builder constructor.

// Custom PCS, Predefined GCS
string customWkt = "PROJCS[\"WebMercatorMile\",GEOGCS[\"GCS_WGS_1984\",DATUM[\"D_WGS_1984\",SPHEROID[\"WGS_1984\",6378137.0,298.257223563]],PRIMEM[\"Greenwich\",0.0],UNIT[\"Degree\",0.0174532925199433]],PROJECTION[\"Mercator_Auxiliary_Sphere\"],PARAMETER[\"False_Easting\",0.0],PARAMETER[\"False_Northing\",0.0],PARAMETER[\"Central_Meridian\",0.0],PARAMETER[\"Standard_Parallel_1\",0.0],PARAMETER[\"Auxiliary_Sphere_Type\",0.0],UNIT[\"Mile\",1609.344000614692]]";

// Builder convenience methods don't need to run on the MCT.
SpatialReference spatialReference = SpatialReferenceBuilder.CreateSpatialReference(customWkt);
// SpatialReferenceBuilder properties
//   spatialReference.Wkt == customWkt
//   spatialReference.Wkid == 0
//   spatialReference.VcsWkid == 0
//   spatialReference.GcsWkid == 4326

SpatialReference gcs = spatialReference.Gcs;
// gcs.Wkid == 4326
// gcs.IsGeographic == true
// spatialReference.GcsWkt == gcs.Wkt

// SpatialReferenceBuilder constructors need to run on the MCT.
ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
{
  using (SpatialReferenceBuilder sb = new SpatialReferenceBuilder(customWkt))
  {
    // do something with the builder

    spatialReference = sb.ToSpatialReference();
  }
});