The gas mass fraction in galaxy clusters is a convenient probe to use in cosmological studies, as it can help derive constraints on a range of cosmological parameters. This quantity is, however, subject to various effects from the baryonic physics inside galaxy clusters, which may bias the obtained cosmological constraints. Among different aspects of the baryonic physics at work, in this paper we focus on the impact of the hydrostatic equilibrium assumption. We analyzed the hydrostatic mass bias B , constraining a possible mass and redshift evolution for this quantity and its impact on the cosmological constraints. To that end, we considered cluster observations of the Planck -ESZ sample and evaluated the gas mass fraction using X-ray counterpart observations. We show a degeneracy between the redshift dependence of the bias and cosmological parameters. In particular we find evidence at 3.8 σ for a redshift dependence of the bias when assuming a Planck prior on Ω m . On the other hand, assuming a constant mass bias would lead to the extremely large value of Ω m  >  0.860. We show, however, that our results are entirely dependent on the cluster sample under consideration. In particular, the mass and redshift trends that we find for the lowest mass-redshift and highest mass-redshift clusters of our sample are not compatible. In addition, we show that assuming self-similarity in our study can impact the results on the evolution of the bias, especially with regard to the mass evolution. Nevertheless, in all the analyses, we find a value for the amplitude of the bias that is consistent with B  ∼ 0.8, as expected from hydrodynamical simulations and local measurements. However, this result is still in tension with the low value of B  ∼ 0.6 derived from the combination of cosmic microwave background primary anisotropies with cluster number counts.