The Washington Post article This black hole is being pushed around its galaxy by gravitational waves also includes an excellent NASA Goddard video description (also in YouTube) of the proposed explanation of the offset of a galaxy's super massive black hole from, and velocity away from the center of the galaxy. The object in this example is Galaxy Cluster, Quasar 3C 186. See also NASA news item Gravitational Wave Kicks Monster Black Hole Out of Galactic Core.
above: The Hubble Space Telescope image that revealed the runaway quasar. (NASA, ESA, and M. Chiaberge/STScI and JHU) From here.
A proposed explanation can be found in the ArXiv preprint Chiaberge et al (2016) The puzzling radio-loud QSO 3C 186: a gravitational wave recoiling black hole in a young radio source?.
[...]Recoiling black holes (BH) may also result from BH-BH mergers and the associated anisotropic emission of gravitational waves (GW, Peres 1962; Beckenstein et al. 1973). The resultant merged BH may receive a kick and be displaced or even ejected from the host galaxy (Merritt et al. 2004; Madau & Quataert 2004; Komossa 2012), a process that has been extensively studied with simulations (Campanelli et al. 2007; Blecha et al. 2011, 2016). Typically, for non-spinning BHs, the expected velocity is of the order of a few hundreds of km s−1, or less. Recent work based on numerical relativity simulations have shown that superkicks of up to ∼ 5000 km s−1 are possible, but are expected to be rare (e.g. Campanelli et al. 2007; Brügmann et al. 2008).
If I understand the proposed explanation correctly, if the galaxy was formed as a merger of two (or more) galaxies, each with a central super massive black hole, and the two black holes merge through spin-down by gravitational radiation, and if they are of unequal masses, the resulting merged black hole can interact with the gravitational waves and receive a "kick", and fly off in one direction rather than remain at the center of mass of the two black holes.
So it seems that gravitational waves can impart linear momentum to objects - but how? What if a wave from an unrelated even was incident on separate black hole, or a start - would it also give them a "kick" - transfer some net linear momentum to them as it passes?