No, to my knowledge it isn't possible to retrieve geometry generated by a geometry shader. Data generated in a graphics pipeline is purged after completion so keeping changes made by a vertex or a tessellation shader isn't possible either. You'll need to make your changes either using a compute shader or on the CPU.
Of course it's possible, that's exactly what transform feedback (stream output in D3D) does.
Oops I didn't know about transform feedback, thanks for correcting me.