I see myself tuning VOR/DME stations enroute and perform cross check.
That is only sensible.
Re the CDI flag, that's interesting, and probably intended behaviour. I originally had a KI-525 HSI whose NAV source could be switched (as is traditional) between GPS (from the GPS) or NAV (VOR/LOC according to the frequency selected on the radio). The Invalid flag was never driven by the GPS, AFAIR. Now I have an EHSI (Sandel 3500) and that makes it very obvious if the GPS is not picking up a signal.
Re GPS failures, I've had them too. One off Italy in 2004, halfway between Brindisi and Dubrovnik, at about 1000ft. Lasted a few minutes, and affected all three units. That one was porb99 jamming. One departing from Padova (Italy) in 2006 which lasted from startup for about 30-45 mins, which affected only the one GPS (KLN94) and that one was probably a corrupted constellation table in the GPS, which always takes a while to clean up. I then had a strange Garmin 496 failure in 2010 when a specific DME frequency was selected, without any other GPS being affected, which is not explained by any subharmonic of 1575MHz.
Out of a total flying time of about 1400hrs that is under 0.1% i.e. a 99.9% + availability. And if you predicate GPS approaches on obvious GPS availability, let alone RAIM, you are looking at a
100% reliability. I've never had a GPS malfunction which wasn't totally obvious on the GPS.
In flight, the possible lack of a CDI flag should not bother the pilot because few would (or should) be flying with GPS using a CDI. That is probably how the "old school" started using GPS, and indeed the original non moving map units (Trimble etc) did only that i.e. generating a cross track error which would be viewed on a CDI. Today, nobody should be flying with a GPS in that way. There is only one thing worse and that is plotting the GPS coordinates on a map
One should be monitoring the
GPS moving map as the very primary source of navigation data. That map is the "big picture". And if you lose signal, that is where any error messages will appear.
Admittedly a Garmin 430 has a map too small to be of much use, which is why I don't have a Garmin 430
And if I did, I would have it feeding a nice size MFD. I don't consider a single GNS430 sized unit as adequate for IFR.