  We present JVLA-C observations of the HI gas in JO204, one of the most
striking jellyfish galaxies from the GASP survey. JO204 is a massive galaxy in
the low-mass cluster Abell 957 at z=0.04243. The HI map reveals an extended 90
kpc long ram-pressure stripped tail of neutral gas, stretching beyond the 30
kpc long ionized gas tail and pointing away from the cluster center. The HI
mass seen in emission is (1.32 $ \pm 0.13) \times 10^{9} \rm M_{\odot}$, mostly
located in the tail. The northern part of the galaxy disk has retained some HI
gas, while the southern part has already been completely stripped and displaced
into an extended unilateral tail. Comparing the distribution and kinematics of
the neutral and ionized gas in the tail indicates a highly turbulent medium.
Moreover, we observe associated HI absorption against the 11 mJy central radio
continuum source with an estimated HI absorption column density of 3.2 $\times
10^{20}$ cm$^{-2}$. The absorption profile is significantly asymmetric with a
wing towards higher velocities. We modelled the HI absorption by assuming that
the HI and ionized gas disks have the same kinematics in front of the central
continuum source, and deduced a wider absorption profile than observed. The
observed asymmetric absorption profile can therefore be explained by a clumpy,
rotating HI gas disk seen partially in front of the central continuum source,
or by ram-pressure pushing the neutral gas towards the center of the continuum
source, triggering the AGN activity.
