GS3 Orchestra, if used on a computer that has an audio card that has GSIF-2 drivers, can accept audio as an input. That audio can be mixed, routed, and processed just like audio generated inside GS3. This is how, for example, you could apply GigaPulse Pro impulses to "external audio" (similar to what GigaPulse VST will be able to do).
In the GS3 Manual, pg. 69, there is "Tutorial 3 - Routing Audio in the DSP Station", which appears to give an example of using external audio.