To be fair, Beagle, the GPS150 and GNC250 are so utterly ghastly that if I was presented with a plane with one of these in it, I wouldn't even bother switching it on.
The moving map alone on these is marginally better than a tuna sandwich
These are 1980s technology.
If I was going to the hassle of installing a GPS in a plane in a permanent manner, I would put in something with a decent usable map, and the
KMD150 is about the baseline there. The Skymap 3C is often panel mounted and is very similar. All of these are way better than the much more expensive Garmin 430, for non-airways usage.