(Garmin is using GR Glide Ratio, not LD. The confusion comes to the fact
that a "glide ratio" relative to the air is not the same to glide ratio
relative to ground.
LD should be used for indicating glide ratio relative to air, and GR or Eff
for ground. But since most pilots don't care at all about the LD (air
relative efficiency) because when we are gliding we are flying toward a
ground point, then we all use either LD GR or EFF.
Old instrument with no GPS are using LD and this is not accounting for wind.
Even bigger confusion)

Months ago I did experiment using current efficiency averaged in last 90
seconds for calculating altitude arrivals (using lk8000, not seeyou).
During long glides it was very accurate.
Currently arrival altitudes are calculated using your MC setting, but if you
fall in sinking air and follow the glider's speed to fly indicator then you
will speed up and use a different MC at all effects.
So now I am experimenting what paraglider's are already using since time:
the "equivalent MC", which is based on your airspeed and polar.
In the end it is like using average efficiency.

Personally I think that it's all a matter of guessing, and the good old GR
or Eff is still the most simple and valuable parameter.

paolo