  We present the first search for $z\geqslant7$, continuum-confirmed sources
with NIRISS/WFS spectroscopy over the Abell 2744 Frontier Fields cluster, as
part of the GLASS-JWST ERS survey. With $\sim15$ hrs of pre-imaging and
multi-angle grism exposures in the F115W, F150W, and F200W filters, we describe
the general data handling (i.e., reduction, cleaning, modeling, and extraction
processes) and analysis for the GLASS-JWST survey. We showcase the power of
JWST to peer deep into reionization, when most intergalactic hydrogen is
neutral, by confirming two galaxies at $z=8.04\pm0.15$ and $z=7.90\pm0.13$ by
means of their Lyman breaks. Fainter continuum spectra are observed in both the
F150W and F200W bands, indicative of blue ($-1.69$ and $-1.33$) UV slopes and
moderately-bright absolute magnitudes ($-20.37$ and $-19.68$ mag). We do not
detect strong Ly$\alpha$ in either galaxy, but do observe tentative
($\sim2.7-3.8\sigma$) HeII$\lambda$1640 A, OIII]$\lambda\lambda$1661,1666 A,
and NIII]$\lambda\lambda$1747,1749 A line emission in one, suggestive of low
metallicity, star-forming systems with possible non-thermal contributions.
These novel observations provide a first look at the extraordinary potential of
JWST/NIRISS for confirming representative samples of bright $z\geqslant7$
sources in the absence of strong emission lines, and gain unprecedented insight
into their contributions towards cosmic reionization.
