Angier,
  I gave an STEC 55 with GPSS steering.  I'm not familiar with "Hi Track" mode, but on my A/P, if you're flying an instrument approach, you need to switch the A/P to approach mode and change the GPSS back to heading mode for steering inside the IAF or I believe the A/P will revert to heading mode at that point.   If you have GPS output going to your CDI, the A/P will fly the approach just like it would tracking a localizer.  Bottom line, GPSS is not used for flying approaches.  I'm not sure if the 30 works the same, but check your ops manual to be sure.
  Skip Slater