From where I fly:
- glider w/ gear up turning into final / on long final: Be nice and give him a radio call
- glider (still?) w/ gear up on short final: Leave him alone. Make sure that the beer is in the cooler.

Biggest damage will be the one to the driver's ego.
Someone who is too distracted to prepare for landing will most probably fail to react to a buzzer or other onboard alarms, so I don't see the point of these gadgets.
And somebody who just forgot to do the check - well, he won't next time.