Actually that is especially easy to do with the GSN. Set the flash for auto at a certain aperture. Then put the camera at 1 to 2 stops smaller than that. As I mentioned, if you set the flash at 5.6, then set the camera at 8 or 11. With the GSN, this is great because the camera then picks the correct shutter speed for 8 or 11. So your ambiant will come out fine, and the flash will fill in the foreground.
It's not as easy with, say, the Canonet because you control the shutter speed, not the aperture.
allan