  Since their serendipitous discovery, Fast Radio Bursts (FRBs) have garnered a
great deal of attention from both observers and theorists. A new class of radio
telescopes with wide fields of view have enabled a rapid accumulation of FRB
observations, confirming that FRBs originate from cosmological distances. The
high occurrence rate of FRBs and the development of new instruments to observe
them create opportunities for FRBs to be utilized for a host of astrophysical
and cosmological studies. We focus on the rare, and as yet undetected, subset
of FRBs that undergo repeated bursts and are strongly gravitationally lensed by
intervening structure. An extremely precise timing of burst arrival times is
possible for strongly lensed repeating FRBs, and we show how this timing
precision enables the search for long wavelength gravitational waves, including
those sourced by supermassive black hole binary systems. The timing of burst
arrival for strongly lensed repeating FRBs is sensitive to gravitational wave
sources near the FRB host galaxy, which may lie at cosmological distances and
would therefore be extremely challenging to detect by other means. Timing of
strongly lensed FRBs can also be combined with pulsar timing array data to
search for correlated time delays characteristic of gravitational waves passing
through the Earth.
