  Although general relativity (GR) has been precisely tested at the solar
system scale, precise tests at a galactic or cosmological scale are still
relatively insufficient. Here, in order to test GR at the galactic scale, we
use the newly compiled galaxy-scale strong gravitational lensing (SGL) sample
to constrain the parameter $\gamma_{PPN}$ in the parametrized post-Newtonian
(PPN) formalism. We employ the Pantheon sample of type Ia supernovae
observation to calibrate the distances in the SGL systems using the Gaussian
Process method, which avoids the logical problem caused by assuming a
cosmological model within GR to determine the distances in the SGL sample.
Furthermore, we consider three typical lens models in this work to investigate
the influences of the lens mass distributions on the fitting results. We find
that the choice of the lens models has a significant impact on the constraints
on the PPN parameter $\gamma_{PPN}$. We use the Bayesian information criterion
as an evaluation tool to make a comparison for the fitting results of the three
lens models, and we find that the most reliable lens model gives the result of
$\gamma_{PPN}=1.065^{+0.064}_{-0.074}$, which is in good agreement with the
prediction of $\gamma_{PPN}=1$ by GR. As far as we know, our 6.4% constraint
result is the best result so far among the recent works using the SGL method.
