  This document presents the concept and physics case for a magnetized gaseous
argon-based detector system (ND-GAr) for the Deep Underground Neutrino
Experiment (DUNE) Near Detector. This detector system is required in order for
DUNE to reach its full physics potential in the measurement of CP violation and
in delivering precision measurements of oscillation parameters. In addition to
its critical role in the long-baseline oscillation program, ND-GAr will extend
the overall physics program of DUNE. The LBNF high-intensity proton beam will
provide a large flux of neutrinos that is sampled by ND-GAr, enabling DUNE to
discover new particles and search for new interactions and symmetries beyond
those predicted in the Standard Model.
