Are there more unconfirmed GR predictions? News says that we have finally observed "gravitational waves", one of GR's predictions. I've read about some other predictions of GR like how gravity affects the flow of time, gravitational lensing and so on. But these predicted phenomenons have all been observed and been proven to exist. 
I'm wondering if there are other predictions of GR that for the time being have not been proven by experiment?
 A: Here are some suggestions of predictions of GR which have been not confirmed yet:
1.) The gravitational force exerted by relativistic sources. In the same way non-relativistic matter produces a force, general relativity predicts that photons (or other particles that move relativistically) exert a force on other particles. Due to experimental constraints this has not been observed yet, although there are some proposals for future experiments.
2.) Gravitational waves (GWs) can slow down time for observers. Since this is a second order effect in the perturbation, this effect is very small and has not been measured yet.
3.) Although the speed of GWs has been constrained from the first binary neutron star merger GW170817, some people claim that there could be a deviation from the speed of light at different frequencies.
4.) Black holes have characteristic frequencies and although there has been some evidence that we have seen them, one would like to measure with greater accuracy in the future.
Depending on your philosophical point of view, it might also be difficult to prove that a certain fact is 'true'. For example, it has been shown that gravitational waves exists, but because the detectors are not accurate enough, we could not verify that the waveform models are exactly as predicted by general relativity. As also was pointed out in the comments, we also do not know the waveforms to all orders. (This perturbation theory is called post-newtonian perturbation theory.)
A: From the top of my head, there are both wormhole and white hole solutions to Einstein's equations that have not been experimentally observed
