Question: Do any gliders come with a ground proximity warning system (GPWS)? I realise that, for a glider, a GPWS probably wouldn't be able to help quite as much as in a powered aircraft, since a glider, by definition, doesn't have any engines which it can apply TOGA power on (unless, of course, it's a motorglider), but it could still warn the pilot if they were descending too rapidly for their altitude or were about to land with the glider in an unsafe configuration, thus allowing the pilot to take the necessary actions for avoiding a crash (for instance, by pulling back on the yoke, jettisoning ballast, lowering the flaps and landing gear if the glider in question is so equipped, etc.).

Answer: 
GPWS is mostly useful for aircraft flying in instrument conditions or at night when there is low visibility preventing a good view of the ground. Gliders [aren't designed to fly in these conditions](https://aviation.stackexchange.com/q/26808/1696) and they don't have the required instruments. They should be able to see the ground and avoid it without GPWS.
I found [this video of a glider equipped with GPWS](https://www.youtube.com/watch?v=sDVCrcs7y8U), however, I doubt that such a system is very useful in glider, that isn't very complex and would consider it a gimmick. 

Also because the sounds in the video are Boeing GPWS sounds, so it's probably self-built for fun.

So, to summarize: There surely are some self-built systems for fun and coolness, but I don't think there would be enough demand and need for a commercial system.