Exploiting broad- and narrow-band images of the Hubble Space Telescope from near-UV to I-band restframe, we study the star-forming clumps of six galaxies of the GASP sample undergoing strong ram-pressure stripping (RPS). Clumps are detected in Hα and near-UV, tracing star formation on different timescales. We consider clumps located in galaxy disks, in the stripped tails and those formed in stripped gas but still close to the disk, called extraplanar. We detect 2406 Hα-selected clumps (1708 in disks, 375 in extraplanar regions, and 323 in tails) and 3750 UV-selected clumps (2026 disk clumps, 825 extraplanar clumps and 899 tail clumps). Only ∼ 15% of star-forming clumps are spatially resolved, meaning that most are smaller than ∼ 140 pc. We study the luminosity and size distribution functions (LDFs and SDFs, respectively) and the luminosity-size relation. The average LDF slope is 1.79 ± 0.09, while the average SDF slope is 3.1 ± 0.5. Results suggest the star formation to be turbulence driven and scale-free, as in main-sequence galaxies. All the clumps, whether they are in the disks or in the tails, have an enhanced Hα luminosity at a given size, compared to the clumps in main-sequence galaxies. Indeed, their Hα luminosity is closer to that of clumps in starburst galaxies, indicating that ram pressure is able to enhance the luminosity. No striking differences are found among disk and tail clumps, suggesting that the different environments in which they are embedded play a minor role in influencing the star formation.