  We present the first James Webb Space Telescope/NIRCam-led determination of
$7<z<9$ galaxy properties based on broadband imaging from 0.8 to 5~$\mathrm{\mu
m}$ as part of the GLASS-JWST Early Release Science program. This is the
deepest dataset acquired at these wavelengths to date, with an angular
resolution $\lesssim0.14$ arcsec. We robustly identify 13 galaxies with
$S/N\gtrsim8$ in F444W from 8 arcmin$^2$ of data at $m_{AB}\leq 28$ from a
combination of dropout and photometric redshift selection. From simulated data
modeling, we estimate the dropout sample purity to be $\gtrsim90\%$. We find
that the number density of these F444W-selected sources is broadly consistent
with expectations from the UV luminosity function determined from Hubble Space
Telescope data. We characterize galaxy physical properties using a Bayesian
Spectral Energy Distribution fitting method, finding median stellar mass
$10^{8.5}M_\odot$ and age 140 Myr, indicating they started ionizing their
surroundings at redshift $z>9.5$. Their star formation main sequence is
consistent with predictions from simulations. Lastly, we introduce an
analytical framework to constrain main-sequence evolution at $z>7$ based on
galaxy ages and basic assumptions, through which we find results consistent
with expectations from cosmological simulations. While this work only gives a
glimpse of the properties of typical galaxies that are thought to drive the
reionization of the universe, it clearly shows the potential of JWST to unveil
unprecedented details on galaxy formation in the first billion years.
