  The Lorentz factor (LF) of gamma-ray burst (GRB) ejecta may be constrained by
observations of high-energy (HE) spectral attenuation. The recent Fermi-LAT
observations of prompt GeV emission from several bright GRBs have leaded to
conclusions of unexpectedly large LFs, $\Gamma>10^3$. Here we revisit this
problem with two main concerns. (1) With one-zone assumption where all photons
are assumed to be generated in the same region (radius) and time, we {\em
self-consistently} calculate the $\gamma\gamma$ optical depth by adopting a
target photon spectrum with HE cutoff. We find that this might be important
when the GRB LF is below a few hundreds. (2) Recent Fermi-LAT observations
suggest that the bulk MeV-range and HE ($\ga100$ MeV) emission may arise from
different regions. We then consider a two-zone case where HE emission is
generated in much larger radii than that of the MeV-range emission. We find
that the HE emission may be mainly attenuated by MeV-range emission and that
the attenuated HE spectrum does not show an exponential spectral cutoff but a
slight steepening. This suggests that there may be no abrupt cutoff due to
$\gamma\gamma$ attenuation if relaxing the one-zone assumption. By studying the
spectra of three bright Fermi-LAT GRBs 080916C, 090510 and 090902B, we show
that a bulk LF of\textbf{ $\Gamma\sim600$ }can be consistent with observations
in the two-zone case. Even lower LFs can be obtained in the multi-zone case.
