Expected flux from the GMC in the Gould Belt vs. their extension (extension is 70% of the boundaries listed in Dame et al. 1987). Expected flux (>100 GeV) is calculated through Eq. (8), with κ = 1. The solid line is the on-axis sensitivity of CTA (>100 GeV, 50 h; Actis et al. 2011), linearly scaled with the extension of the source. This optimistic scaling holds for an infinite field of view with flat acceptance, while we expect a degradation of sensitivity with increasing angular distance from the center of the field of view.