ftrplt has explained it quite clearly. Don't just read the Garmin users guide to figure out what your aircraft is capable of, it depends upon how the Garmin was configured when installed. Your AFM will tell you what the aircraft is equipped for. Look in the limitations section, or the supplements section, and it will tell you if it's suitable for Baro VNAV, or LPV. If there are none of these statements then you must fly to the LNAV MDA only, you may use the vertical guidance as ADVISORY only (coupled or non coupled) to the MDA. If there are statements about Baro-VNAV/LPV, there will be associated limitations, abnormal and normal procedures you must follow.
The reason for the training is to cover precisely what has been discussed here, to ensure the pilot is aware of what approaches/minima the aircraft can/can't be flown to, and how to fly them. It's not just a CASA requirement Dick, FAA and EASA require it too. On my last re-current in the US we had to cover avionics differences, and guess what? We covered Baro VNAV and LPV, practiced using the temp comp system, identified the different annunciations for each approach. And the fact that for each different type of vertical guidance (ILS, VPATH, VGP, and LPV), the vertical path had a different symbol!!! So the training is very important because it ensures the pilot knows what he/she is looking at.