  The first multimessenger observation of a neutron star merger was
independently detected in gamma-rays by Fermi-GBM and INTEGRAL SPI-ACS and
gravitational waves by Advanced LIGO and Advanced Virgo. Gravitational waves
are emitted from systems with accelerating quadrupole moments, and detectable
sources are expected to be compact objects. Nearly all distant astrophysical
gamma-ray sources are compact objects. Therefore, serendipitous observations of
these two messengers will continue to uncover the sources of gravitational
waves and gamma-rays, and enable multimessenger science across the Astro2020
thematic areas. This requires upgrades to the ground-based gravitational wave
network and ~keV-MeV gamma-ray coverage for observations of neutron star
mergers, and broadband coverage in both gravitational waves and gamma-rays to
monitor other expected joint sources.
