The metric GS EU Active % represents the percentage of overall GPU time that the EUs were actively executing Geometry Shader instructions.
If GS EU Active % is 50% it means that half of the overall GPU time was spent actively executing geometry shader instructions.
If GS EU Active % is 0% it means that no geometry shader was associated with the selected draw calls, or that the amount of time actively executing geometry shader instructions was negligible.
This metric is important if geometry shader seems to be the bottleneck for selected rendering calls. If GS EU Active % accounts for most of the EU active time, then to improve performance you may need to simplify the geometry shader or simplify and optimize the geometry of the scene.
If GS EU Active % 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.