As Ken said, the gravitational energy (from a Newtonian POV) is indeed released when systems collapse. The things Ken mentioned are "regular" ways energy can be released -- light, heat, shrapnel, etc.

However, General Relativity predicts orbiting systems can shed energy via gravitational radiation as well. Normally that is miniscule, save for some very exotic, highly relativistic cases like two inspiralling black holes, where an enormous amount of gravitational radiation is predicted.

What fascinates me there is in such cases, the radiation is not isotropic and has a net linear momentum, giving the merged result a big radiation recoil kick. That kick can apparently be large, on the order of 100km/s or tow relative to original center of mass. They think it can well kick it out of the host galaxy and are looking for signatures of that. That is definitely a non Newtonian result!


-Richard