QGPropagator[psi[index1, momentum], psibar[index2, momentum], mass] is a placeholder for the propagator of the field psi. It is introduced in the QGARF style file "feyncalc.sty".
QGPropagator[psi[index1, momentum], psibar[index2, momentum], mass]
psi
"feyncalc.sty"
Overview.