The Post-GS Primitives metric 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 the GS was not enabled for the selected rendering calls, the metric returns a value of 0.
If Post-GS Primitives is 1000 and Primitive Count is 100, it means that 1000 primitives were constructed in the geometry shader from the original 100.
Analyze the geometry shader code using Graphics Frame Analyzer.