γ-ray fluxes expected from a GMC with A = 1 compared to the point-like source sensitivities of currently operating (left) and future (right) γ-ray instruments. On the left: the ten-year Fermi-LAT sensitivity for the outer (light red) and inner Galaxy (dark red); the H.E.S.S. sensitivity for 100-h observations with the four-telescope system configuration (solid yellow) and for 50 h with the five-telescope configuration (dashed yellow); the HAWC five-year sensitivity (green); and the LHAASO one-year sensitivity (solid violet). On the right: the sensitivity for a hypothetical Very Large Area Telescope (VLAT) with a three-times-larger effective area (blue curves); the expected sensitivity of CTA from the northern (light orange) and southern (orange) sites, for 50 h of observations; the sensitivity of SWGO (cyan) after five years of observations; and the sensitivity of LHAASO (violet) for five years of observations. The black solid line is the flux of a MC of A = 1 illuminated by the CR sea. The two spectra above 10⁴ GeV represent two options of CR proton spectra based on the spectra reported above 10⁶ GeV by the KASCADE (dashed line) and ICEtop (dotted line) collaborations (see main text). As a reference, the fluxes representing 10, 1, and 0.1% of the γ-ray flux from the Crab Nebula (Cao et al. 2021) are shown.