The unit cant "act" as a HSI however it is a requirement of an IFR instalation that the command bar is linked to a CDI type indicator of the type that you would normaly find with a VOR ......or a HSI.
To save panel space if you have an ARINC vor indicator the GPS can be switched to it , a number of companys produce the switch gear intigrated with the anunciators that are required for IFR flight.
The switch gear that seems to be in favor at the moment is made by mid-continent or ameri-king i personaly favor the ameri-king unit that you can change the lamps on but both these companys produce a range of good products.
Also dont forget that the GPS should be conected to the transponder alt encoder as this is a requirment for the GPS APP mode.
[ 10 October 2001: Message edited by: A and C ]