The metric Post-GS Primitives represents the number of primitives that flowed out of the geometry shader (GS) (if enabled) to the clipper. This metric is important if a geometry shader was associated with the selected rendering calls, and even more important if the number of primitives spawned by geometry shader code is dynamic.
If GS was not enabled for selected rendering calls the metric will return a value of 0.
If Post-GS Primitives is 1000 and Primitive Count is 100 it means that 1000 primitives were constructed in geometry shader from the original 100.
Use the Intel® GPA Frame Analyzer Shaders Tab to examine geometry shader code.