  One of the most fundamental baryonic matter components of galaxies is the
neutral atomic hydrogen (H$\,{\rm \scriptsize I}$). At low redshifts, this
component can be traced directly through the 21-cm transition, but to infer
H$\,{\rm \scriptsize I}$ gas content of the most distant galaxies, a viable
tracer is needed. We here investigate the fidelity of the fine structure
transition of the ($^2P_{3/2} - ^2P_{1/3}$) transition of singly-ionized carbon
[C$\,{\rm \scriptsize II}$] at $158\,\mu$m as a proxy for H$\,{\rm \scriptsize
I}$ in a set simulated galaxies at $z\approx 6$, following the work by Heintz
et al. (2021). We select 11,125 star-forming galaxies from the SIMBA
simulations, with far-infrared line emissions post-processed and modeled within
the SIGAME framework. We find a strong connection between [C$\,{\rm \scriptsize
II}$] and H$\,{\rm \scriptsize I}$, with the relation between this [C$\,{\rm
\scriptsize II}$]-to-H$\,{\rm \scriptsize I}$ relation ($\beta_{\rm [C\,{\rm
\scriptsize II}]}$) being anti-correlated with the gas-phase metallicity of the
simulated galaxies. We further use these simulations to make predictions for
the total baryonic matter content of galaxies at $z\approx 6$, and specifically
the HI gas mass fraction. We find mean values of $M_{\rm HI}/M_\star = 1.4$,
and $M_{\rm HI}/M_{\rm bar,tot} = 0.45$. These results provide strong evidence
for H$\,{\rm \scriptsize I}$ being the dominant baryonic matter component by
mass in galaxies at $z\approx 6$.
