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: 
Many glider pilots have a GPS flight computer that usually has terrain loaded in its database and can alert a glider pilot of terrain. This is not quite a GPWS but is cheaper and uses a lot less power (glider pilots rely on batteries for their electrical system). Most GA aircraft don't have a GPWS either and usually rely on the GPS solution. ForeFlight has a nice feature called Synthetic Vision which can allow you to see terrain in IMC conditions and is much cheaper than a 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.