  We present observations of the $z\sim5.7$ Lyman-break galaxy HZ10 with the
JWST/NIRSpec IFU in high and low spectral resolution (G395H, $R\sim2700$ and
PRISM, $R\sim100$, respectively), as part of the GA-NIFS program. By spatially
resolving the source, we find evidence for three spatially and spectrally
distinct regions of line emission along with one region of strong continuum
emission, all within a projected distance of $<10$kpc. The R2700 data features
strong detections in H$\beta$, [OIII]$\lambda\lambda4959{,}5007$,
[NII]$\lambda\lambda6548{,}6584$, H$\alpha$, and
[SII]$\lambda\lambda6716{,}6731$. The R100 data additionally contains a strong
detection of the Ly$\alpha$ break, rest-UV continuum, and
[OII]$\lambda\lambda3726{,}3729$. None of the detected lines present strong
evidence for AGN excitation from line diagnostic diagrams, and no
high-ionisation lines are detected. Using the detected lines, we constrain the
electron density $\left( \rm \log_{10}\left( n_e /
cm^{-3}\right)\sim2.5-3.3\right)$ and metallicity ($\sim0.5-0.7$ solar) in each
component. Spaxel-by-spaxel fits of each cube reveal a strong east-west
velocity gradient and significant line asymmetries (indicating tidal features
or outflows). The western component features a very red UV slope
($\beta_{UV}\sim-1$) and significant H$\alpha$ emission, suggesting an evolved
population and active star formation. From a comparison to high resolution
[CII]$158\mu$m imaging obtained with the Atacama Large Millimetre/submillimetre
Array (ALMA), we find that the continuum emitter is associated with a gas-poor
stellar population. Altogether, these data suggest that HZ10 represents an
ongoing merger, with a complex distribution of stars, gas, and dust $<1$Gyr
after the Big Bang.
