  Knowledge of the bulk Lorentz factor $\Gamma_{0}$ of GRBs allows us to
compute their comoving frame properties shedding light on their physics. Upon
collisions with the circumburst matter, the fireball of a GRB starts to
decelerate, producing a peak or a break (depending on the circumburst density
profile) in the light curve of the afterglow. Considering all bursts with known
redshift and with an early coverage of their emission, we find 67 GRBs with a
peak in their optical or GeV light curves at a time $t_{\rm p}$. For another
106 GRBs we set an upper limit $t_{\rm p}^{\rm UL}$. We show that $t_{\rm p}$
is due to the dynamics of the fireball deceleration and not to the passage of a
characteristic frequency of the synchrotron spectrum across the optical band.
Considering the $t_{\rm p}$ of 66 long GRBs and the 85 most constraining upper
limits, using censored data analysis methods, we reconstruct the most likely
distribution of $t_{\rm p}$. All $t_{\rm p}$ are larger than the time $t_{\rm
p,g}$ when the prompt emission peaks, and are much larger than the time $t_{\rm
ph}$ when the fireball becomes transparent. The reconstructed distribution of
$\Gamma_0$ has median value $\sim$300 (150) for a uniform (wind) circumburst
density profile. In the comoving frame, long GRBs have typical isotropic
energy, luminosity, and peak energy $\langle E_{\rm iso}\rangle=3(8)\times
10^{50}$ erg, $\langle L_{\rm iso}\rangle=3(15) \times 10^{47}$ erg s$^{-1}$ ,
and $\langle E_{\rm peak}\rangle =1(2)$ keV in the homogeneous (wind) case. We
confirm that the significant correlations between $\Gamma$ and the rest frame
isotropic energy ($E_{\rm iso}$), luminosity ($L_{\rm iso}$) and peak energy
($E_{\rm peak}$) are not due to selection effects. Assuming a typical opening
angle of 5 degrees, we derive the distribution of the jet baryon loading which
is centered around a few $10^{-6} {\rm M_{\odot}}$.
