We present VLA H I observations of JO206, a prototypical ram-pressure- stripped galaxy in the GASP sample. This massive galaxy (M* = 8.5 × 1010 M☉) is located at a redshift of z = 0.0513, near the centre of the low-mass galaxy cluster, IIZw108 (σ ̃ 575 km s-1). JO206 is characterized by a long tail (≥90 kpc) of ionized gas stripped away by ram pressure. We find a similarly long H I tail in the same direction as the ionized gas tail and measure a total H I mass of 3.2 × 109 M☉. This is about half the expected H I mass given the stellar mass and surface density of JO206. A total of 1.8 × 109 M☉ (60 per cent) of the detected H I is in the gas-stripped tail. An analysis of the star formation rate shows that the galaxy is forming more stars compared to galaxies with the same stellar and H I mass. On average we find an H I gas depletion time of ̃0.5 Gyr which is about four times shorter than that of `normal' spiral galaxies. We performed a spatially resolved analysis of the relation between star formation rate density and gas density in the disc and tail of the galaxy at the resolution of our H I data. The star formation efficiency of the disc is about 10 times higher than that of the tail at fixed H I surface densities. Both the inner and outer parts of JO206 show an enhanced star formation compared to regions of similar H I surface density in field galaxies. The enhanced star formation is due to ram-pressure stripping during the galaxy's first infall into the cluster.