I was in a thermal near my home airport with two other gliders; we were spread out about 500' vertical from each other. A Cessna 172 breezed through the centre of our group, maybe 75 metres from me.

Thank you for sharing this with us. This supports my personal strong conviction that the glider ADS-B exemption doesn't make much sense. Either everybody, yes, any aircraft - power, gliders, balloons, drones - is equipped or the system is lame by definition. I don't mention geese, hawks and other birds because they do quite a good job avoiding us with our low speeds, which they sometimes can't do when challenged by airliners.

BTW, is there any talk about placing the ADS-B signals on high towers marked on aviation maps? That would be an interesting add-on to the system.