The eTrex 20 does indeed have the Glide Ratio field.
The mental gymnastics of converting that into something familar, such as per cent gradient, would be too much for me, I think. Besides, the gradient isn’t all that interesting to me. What is more interesting is vertical speed. The eTrex has a Vertical Speed field too, but the units are not the familiar metres per hour of VAM. Metres per minute is the closest option, I think. Maybe I’ll try that, though who knows if it’s accurate enough to be useful on an eTrex 20. The eTrex 30 would fare better here.