The GAST D version of GBAS that supports Category II and III precision approach and landing operations adds additional monitoring to both the ground and airborne segments in order to minimize the differential errors that can be caused by anomalous ionospheric gradients. Extensive analysis and simulation work was conducted by a team within the International Civil Aviation Organization (ICAO) Navigation Systems Panel (NSP) to validate the performance requirements for ionospheric gradient mitigation. The analysis was specifically designed to confirm that the requirements could be met by the combination of ground and airborne monitors. This paper provides an overview of the validation activity. It includes a description of the design choices and monitors that make up the GBAS GAST D defense against anomalous ionospheric spatial gradients and the analysis and simulation results that demonstrate that the system successfully meets the performance requirements.