  The diffuse ionized gas (DIG) is an important component of the interstellar
medium that can provide insights into the different physical processes
affecting the gas in galaxies. We utilise optical IFU observations of 71
gas-stripped and control galaxies from the Gas Stripping Phenomena in galaxies
(GASP) survey, to analyze the gas properties of the dense ionized gas and the
DIG, such as metallicity, ionization parameter log(q), and the difference
between the measured $\log[OI]/H\alpha$ and the value predicted by star-forming
models, given the measured log[OIII]/H$\beta$ ($\Delta log[OI]/H\alpha$). We
compare these properties at different spatial scales, among galaxies at
different gas-stripping stages, and between disks and tails of the stripped
galaxies. The metallicity is similar between the dense gas and DIG at a given
galactocentric radius. The log(q) is lower for DIG compared to dense gas. The
median values of log(q) correlate best with stellar mass, and the most massive
galaxies show an increase in log(q) toward their galactic centers. The DIG
clearly shows higher $\Delta log[OI]/H\alpha$ values compared to the dense gas,
with much of the spaxels having LIER/LINER like emission. The DIG regions in
the tails of highly stripped galaxies show the highest $\Delta
log[OI]/H\alpha$, exhibit high values of log(q) and extend to large projected
distances from star-forming areas (up to 10 kpc). We conclude that the DIG in
the tails is at least partly ionized by a process other than star-formation,
probably by mixing, shocks and accretion of inter-cluster and interstellar
medium gas.
