Astro2020 Science White Paper: The Extended Cool Gas Reservoirs Within z
  > 1 (Proto-)Cluster Environments
  High-redshift ($z$) proto-clusters will serve as testing grounds to probe the
gas supply furnishing the emerging metals, stars, and large-scale structures we
see at the current epoch. This work focuses on the major role large
radio/millimeter (mm) single dish facilities will have in constraining the
bulk, cold (T $= 10^{1-4}$K) molecular and atomic gas content. To highlight the
need for large radio/mm single dishes, we calculate how the high-sensitivity of
the Green Bank Telescope's (GBT) unblocked 100m aperture provides vital
interferometric short-spacing coverage to support higher-resolution ngVLA
observations of the cold neutral gas at the largest scales. These combined
observations are optimal for revealing low-surface brightness emission, and
thus aid in the total baryonic mass estimates across cosmic time.
