  We present a study of the physical properties of JO201, a unique disk galaxy
with extended tails undergoing extreme ram-pressure stripping as it moves
through the massive cluster Abell 85 at supersonic speeds mostly along the line
of sight. JO201 was observed with MUSE as part of the GASP programme. In a
previous paper (GASP II) we studied the stellar and gas kinematics. In this
paper we present emission-line ratios, gas-phase metallicities and ages of the
stellar populations across the galaxy disk and tails. We find that while the
emission at the core of the galaxy is dominated by an active galactic nucleus
(AGN), the disk is composed of star-forming knots surrounded by excited diffuse
gas. The collection of star-forming knots presents a metallicity gradient
steadily decreasing from the centre of the galaxy outwards, and the ages of the
stars across the galaxy show that the tails formed <10^9 yr ago. This result is
consistent with an estimate of the stripping timescale (1 Gyr), obtained from a
toy orbital model. Overall, our results independently and consistently support
a scenario in which a recent or ongoing event of intense ram-pressure stripping
acting from the outer disk inwards, causes removal and compression of gas, thus
altering the AGN and star-formation activity within and around the galaxy.
