Central brightness distribution of spiral disk in the samples of Kent (1985a), Courteau (1996), MacArthur et al. (2003) and Gadotti (2009). The two dashed lines corresponds to the limiting region containing spiral disks having absolute G magnitudes of −18 and −22 mag. The solid inclined line corresponds to the limiting surface brightness magnitude for object detection in the SM section, at a fiducial distance of 10 Mpc. Objects below that line have too low a central surface brightness to be detected by Gaia. As we can observe none of the observed pure exponential disks are in a state to be sampled by Gaia.