I have to report that saying DO-178 is non-specific is kind of like saying that FAR Part 23 is non-specific about certifying airframes. DO-178 is up to rev C and requires full documentation of system requirements, design documentation that describes how the software implementation supports the requirements, unit, integration, and full system test with documentation of results. Winds up being a pretty big paper package that describes how your hardware and software does what you say it does, reliably and repeatably, every time. The paperwork, and particularly the test documentation, is a pretty big burden for a hobbyist who has to have a day job.

You can build a GPS based navigation system around the available components. Fly VFR and have fun. For IFR Garmin has a big head start on the rest of us. The regulatory burden is a big barrier to folks who want to compete with them. And in all fairness, they do what they do really well and their gear is well worth the $$.

The medical device world I live in is very similar. 21 CFR 820 if you need some light bed-time reading.

Best of luck,

Wes