Garmin as Navigator, GRT as a Display....
Just to make sure that people don't misunderstand me, here's how I consider the various parts of my system:
My GRT is an outstanding attitude platform and navigational display. I ave done a LOT of research, including what EAA has to say on the subject, and I do not believe that the display for the GPS system needs to be TSO'd. Some will disagree - forever - and some will agree.( As a matter of fact, you can probably find two FSDO's that would disagree and get them fighting between each other - might be fun!) The GRT also fully integrates systems monitoring functions and fuel calculations, as well as adding situational awareness calculations based on navigation and systems data (such as fuel state at various waypoints, winds aloft affect on ETA's, MPG, SFC, etc....) to increase situational awareness. In addition, it provides some very well thought out display methods for instrument approach guidance (Highway in the Sky) along with more traditional (needles o bars) guidance. Fly the HITS once, and you'll know how easy it is - and since it is driven from the exact same data as the needles, it is plenty reliable.
My Garmin 430 is a fully TSO-compliant and certified GPS navigator which uses the GRT to display it's data.
My TruTrak Pictorial Pilot is a great autopilot which integrates exceptionally well with the GRT EFIS and serves (as well) as a back-up flight instrument and attitude platform. For additional redundancy, I have a second GPS (non-IFR) which can drive the EFIS in an emergency situation (the 430 goes up in smoke for instance).
On top of all of these devices, my Garmin 396 provides yet a third GPS (non-IFR certified) and all of the valuable weather information.
The 430, GRT, and 396 all provide navigation information, the 430 being the only legal one for IFR, and the 396 being second in usability. Of course, that is at the current time. Software is always being updated and improved for the GRT. One nice feature f the GRT is that I can run an internal Flight plan that is different from the external plan provided by the 430, and compare the two for alternate routings.
It's all about situational awareness - the GRT provides and excellent system approach to integrating all of the available data - and makes a great display for the 430 data!
Paul