HSI is a navigational device. It takes you somewhere. But you need to tell it where you want to be taken. On a NAV radio this is a radial, on a GPS this is a direct-to to a waypoint.
The GPS-250 is a position only device. It has no database, and no way to tell it to take you somewhere. If you hooked an GPS-250 to the D100 it wouldn't give you an HSI either, since all the GPS-250 outputs is your current position, not a desired future position.
The next release of the SkyView software adds direct-to flight planning. Technically it can do this from any GPS, including the SV-250. Once you can do a direct-to on the SkyView, then the path to that waypoint will appear on the HSI.
