How about a mechanical warning, like I use on my present glider? It's a
small spring-loaded plastic clamp that is on the gear handle when the
gear is down, then moved to the spoiler handle just before raising the
gear. The clamp location is reversed for landing.

Foolproof... almost - and I'm a better fool. It was a ridge day and the
clouds were barely above the ridge. I had to use spoilers to keep from
getting sucked into the clouds. I got used to working the spoilers with
the clamp attached. Add to that, the failure to use a proper
checklist... No damage at all as it was on grass but sooo many witnesses
:-).

I'm much better at checklists now and also have an audible gear warning.
The warning buzzer is a great idea, and suggest that the BGA
reconsider their opinion.

Tony V.