Check the Giga manual, page 90, "How to ReWire GigaStudio 3.0 with Sonar."
If everything is installed successfully, it should be pretty easy. GS3 shows up in your Insert menu as a ReWire instrument. You can add audio tracks in SONAR to receive multiple outputs from GS3. You render the audio into SONAR the same as you do with your VSTs and DXis.
This may only apply if you're working with SONAR 3 or better. I don't know that the earlier versions had the ReWire capabilities.