  Jellyfish galaxies in clusters are key tools to understand environmental
processes at work in dense environments. The advent of Integral Field
Spectroscopy has recently allowed to study a significant sample of stripped
galaxies in the cluster environment at z$\sim 0.05$, through the GAs Stripping
Phenomena in galaxies with MUSE (GASP) survey. However, optical spectroscopy
can only trace the ionized gas component through the H$_{\alpha}$ emission that
can be spatially resolved on kpc scale at this redshift. The complex interplay
between the various gas phases (ionized, neutral, molecular) is however yet to
be understood. We report here the detection of large amounts of molecular gas
both in the tails and in the disks of 4 jellyfish galaxies from the GASP sample
with stellar masses $\sim 3.5\times 10^{10}-3\times 10^{11} M_{\odot}$, showing
strong stripping. The mass of molecular gas that we measure in the tails
amounts to several $10^9 M_{\odot}$ and the total mass of molecular gas ranges
between 15 and 100 \% of the galaxy stellar mass. The molecular gas content
within the galaxies is compatible with the one of normal spiral galaxies,
suggesting that the molecular gas in the tails has been formed in-situ. We find
a clear correlation between the ionized gas emission $\rm H\alpha$ and the
amount of molecular gas. The CO velocities measured from APEX data are not
always coincident with the underlying $\rm H\alpha$ emitting knots, and the
derived Star Formation Efficiencies appear to be very low.
