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.
A glider will be usually flown according to VFR. So it is basically the task of the pilot to maintain adeqaute distance to terrain. The distance can be rather low, flying in mountains. As already mentioned, current onboard flight computers include terrain databases, that will provide you with AGL. Software like [XCSoar](https://www.xcsoar.org/) will perform a final glide calculation taking terrain into account, telling the pilot if glide path will be in conflict with terrain.
More important than GPWS is obstacle collision avoidance, as obstacles like power lines or cablecars in mountains are hard to see. The popular traffic awareness system [Flarm](https://flarm.com/) can optionally host a obstacle database and then will provide apropriate warnings.