  In our previous work in Xiao et al. (2019), we suggested that 6 superluminal
sources could be gamma-ray candidates, and in fact 5 of them have been
confirmed in the fourth Fermi-LAT source catalogue (4FGL). In this work, based
on the 4FGL, we report a sample of 229 Fermi detected superluminal sources
(FDSs) including 40 new FDSs and 62 non-Fermi detected superluminal sources
(non-FDSs). Thus, we believe that all superluminal sources should have
$\gamma$-ray emissions, and superluminal motion could also be a clue to detect
$\gamma$-ray emission from active galactic nuclei (AGN). We present a new
approach of Doppler factor estimate through the study of the $\gamma$-ray
luminosity ($L_{\rm \gamma}$) and of the viewing angle ($\phi$).
