  Gamma-rays, the most energetic photons, carry information from the far
reaches of extragalactic space with minimal interaction or loss of information.
They bring messages about particle acceleration in environments so extreme they
cannot be reproduced on earth for a closer look. Gamma-ray astrophysics is so
complementary with collider work that particle physicists and astroparticle
physicists are often one in the same. Gamma-ray instruments, especially the
Fermi Gamma-ray Space Telescope, have been pivotal in major multi-messenger
discoveries over the past decade. There is presently a great deal of interest
and scientific expertise available to push forward new technologies, to plan
and build space- and ground-based gamma-ray facilities, and to build
multi-messenger networks with gamma rays at their core. It is therefore
concerning that before the community comes together for planning exercises
again, much of that infrastructure could be lost to a lack of long-term
planning for support of gamma-ray astrophysics. Gamma-rays with energies from
the MeV to the EeV band are therefore central to multiwavelength and
multi-messenger studies to everything from astroparticle physics with compact
objects, to dark matter studies with diffuse large scale structure. These goals
and new discoveries have generated a wave of new gamma-ray facility proposals
and programs. This paper highlights new and proposed gamma-ray technologies and
facilities that have each been designed to address specific needs in the
measurement of extreme astrophysical sources that probe some of the most
pressing questions in fundamental physics for the next decade. The proposed
instrumentation would also address the priorities laid out in the recent
Astro2020 Decadal Survey, a complementary study by the astrophysics community
that provides opportunities also relevant to Snowmass.
