The metric GS EU Stall % represents the percentage of overall GPU time that the EUs were stalled in Geometry Shader instructions.
This metric does not include the total amount of stalled time in the geometry shader but only the fraction of time when the geometry shader stalls were causing the entire EU to stall. The entire EU stalls when all of its threads are stalled.
If GS EU Stall % is 50%, it means that half of the overall GPU time was spent stalled on Geometry Shader instructions.
If GS EU Stall % is 0%, it means that no Geometry Shader was associated with selected rendering calls or Geometry Shader threads were not causing EUs stalls.
This metric will be important if you think that geometry shader looks like the bottleneck for selected rendering calls. If GS EU Stall % accounts for most of the EU active time, then to improve performance you may need to simplify the geometry shader or simplify and optimize geometry.
If GS EU Stall % is more than a nominal amount, you may need to examine your geometry shader code to find reasons for what might be causing these stalls.
Inspect the shader code using the Graphics Frame Analyzer.