Our Garmins are either GPS150 or GNC250, the GPS elements of which are very similar. They even have common user waypoints, whichare listed in the Club FOB.....
Moreover, we even ask that people don't fiddle with the 'standard' set up, just leave it with our aerodrome as the reference, so that brg/rng from home is always available, plus DTK, ETA, GS and XTK on the CDI. That way, a positive cross-check against ruler-measured track, plus computed GS and ETA is available.
But the number of times I've found TRK and DTK left displayed - and met people who think that if DTK = TRK, then they must be on track.... Clearly they've never heard of parallel lines!
However, much of the problem is indeed that many FIs don't understand the GPS!