  We use K-band multi-object near-infrared spectroscopy with Keck/MOSFIRE to
search for environmental imprints on the gas properties of 27 narrow-band
selected H$\alpha$ emitters (HAEs) across the three major clumps of the
assembling USS1558--003 protocluster at $z=2.53$. We target the H$\alpha$ and
[NII]$\lambda$6584 emission lines to obtain star-formation rates (SFR) and
gas-phase oxygen abundances for our sources, confirming the membership of 23
objects. HAEs belonging to this protocluster display enhanced SFRs with respect
to the main sequence of star formation at the same cosmic epoch. This effect is
more prominent for low-mass galaxies ($\mathrm{\log M_*/M_\odot<10.0}$), which
may be experiencing a vigorous phase of mass assembly shortly after they were
formed. We compute the individual and stacked gas-phase metallicities for our
sources finding a metallicity deficit for low-mass objects when compared
against the field mass-metallicity relation and the massive Spiderweb
protocluster at $z=2.16$. These results suggest that HAEs within USS1558--003
may be less evolved than those in the Spiderweb protocluster. Finally, we
explore the gas metallicity - gas fraction relation for a small sample of five
galaxies with CO(3-2) molecular gas information. Assuming our objects are in
equilibrium, we obtain a relatively wide range of mass loading factors
($\mathrm{\lambda=0.5-2}$) matching field samples at the cosmic noon but in
contrast with our previous results in the Spiderweb protocluster. We speculate
that these discrepancies between protoclusters may be (partly) driven by
differences in their current dynamical and mass assembly stages, hinting at the
co-evolution of protoclusters and their galaxy populations at $2<z<3$.
