  We present a joint analysis of the galaxy S04590 at $z=8.496$ based on
NIRSpec, NIRCam, and NIRISS observations obtained through as part of Early
Release Observations programme of the James Webb Space Telescope (JWST) and the
far-infrared [CII]-$158\mu$m emission line detected by dedicated Atacama Large
Millimeter/submillimeter Array (ALMA) observations. We determine the physical
properties of S04590 from modelling of the spectral energy distribution (SED)
and through the redshifted optical nebular emission lines detected with
JWST/NIRSpec. The best-fit SED model reveals a low-mass ($M_\star =
10^{7.2}-10^{8}\,M_{\odot}$) galaxy with a low oxygen abundance of $12+\log{\rm
(O/H)} = 7.16^{+0.10}_{-0.12}$ derived from the strong nebular and auroral
emission lines. Assuming that [CII] effectively traces the interstellar medium
(ISM), we estimate the total gas mass of the galaxy to be $M_{\rm gas} =
(8.0\pm 4.0)\times 10^{8}\,M_\odot$ based on the luminosity and spatial extent
of [CII]. This yields an exceptionally high gas fraction, $f_{\rm gas} = M_{\rm
gas}/(M_{\rm gas} + M_\star) \gtrsim 90\%$, though still consistent within the
range expected for its low metallicity. We further derive the metal mass of the
galaxy based on the gas mass and gas-phase metallicity, which we find to be
consistent with the expected metal production from Type II supernovae. Finally,
we make the first constraints on the dust-to-gas (DTG) and dust-to-metals (DTM)
ratios of galaxies in the epoch of reionization at $z\gtrsim 6$, showing
overall low mass ratios of logDGT $<-3.8$ and logDTM $<-0.5$, though consistent
with local scaling relations and in particular the local metal-poor galaxy I
Zwicky 18. Our analysis highlights the synergy between ALMA and JWST in
characterizing the gas, metal, and stellar content of the first generation of
galaxies.
