  We report the first gas-phase metallicity map of a distant galaxy measured
with the James Webb Space Telescope (JWST). We use the NIRISS slitless
spectroscopy acquired by the GLASS Early Release Science program to spatially
resolve the rest-frame optical nebular emission lines in a gravitationally
lensed galaxy at $z=3.06$ behind the Abell 2744 galaxy cluster. This galaxy
(dubbed GLASS-Zgrad1) has stellar mass $\sim10^{8.6} M_\odot$, instantaneous
star formation rate $\sim8.6$ $M_\odot$/yr (both corrected for lensing
magnification), and global metallicity one-fourth solar. From its emission line
maps ([O III], H$\beta$, H$\gamma$, [Ne III], and [O II]) we derive its spatial
distribution of gas-phase metallicity using a well-established forward-modeling
Bayesian inference method. The exquisite resolution and sensitivity of
JWST/NIRISS, combined with lensing magnification, enable us to resolve this
$z\sim3$ dwarf galaxy in $\gtrsim$50 resolution elements with sufficient
signal, an analysis hitherto not possible. We find that the radial metallicity
gradient of GLASS-Zgrad1 is strongly inverted (i.e. positive): $\Delta\log({\rm
O/H})/\Delta r$ = $0.165\pm0.023$ $\mathrm{dex~kpc^{-1}}$. This inverted
gradient may be due to tidal torques induced by a massive nearby ($\sim$15 kpc
projected) galaxy, which can cause inflows of metal-poor gas into the central
regions of GLASS-Zgrad1. These first results showcase the power of JWST
wide-field slitless spectroscopic modes to resolve the mass assembly and
chemical enrichment of low-mass galaxies in and beyond the peak epoch of cosmic
star formation ($z\gtrsim2$). Reaching masses $\lesssim 10^9~M_\odot$ at these
redshifts is especially valuable to constrain the effects of galactic feedback
and environment, and is possible only with JWST's new capabilities.
