What Happens When A Gravitational Wave Interacts With Another One? If two gravitational waves came in contact with each other what would happen? In another question entirely, what happens when a higher gravitational field interacts with a weaker one.
 A: Weak gravitational waves are just like light or sound: they temporally "interfere" and pass through each-other unaffected. However, if the waves are strong they will "pull" on each-other, since the waves contain energy (which is mass). In the extreme case this would make a black hole. In general, large amplitude waves of anything (light, sound, water, etc) begin to affect each-other in strange ways (the scientific term is "non-linearity"), 
For the stronger vs weaker field, the stronger wave(s) determine the metric and the weaker waves would behave just like light waves would: being "pulled" toward the stronger ones.
"Strong" means that space-time is stretched/squeezed significantly, which requires the mass-energy contained in the waves to approach that of a black hole the same size. This is VERY strong, so don't expect natural gravitational wave interactions to produce black holes. 
Finally, the waves must be moving in different directions. Otherwise you could find a reference frame for which the waves are redshifted to low energies. The same goes for any wave that is being propagated in a vacuum.
