  We study the gas mass fraction, $f\_{\rm gas},$ behavior in $XMM-Newton$
$\Omega$ project. The typical $f\_{\rm gas}$ shape of high redshift galaxy
clusters follows the global shape inferred at low redshift quite well. This
result is consistent with the gravitational instability picture leading to self
similar structures for both the dark and baryonic matter. However, the mean
$f\_{\rm gas} in distant clusters shows some differences to local ones,
indicating a departure from strict scaling. This result is consistent with the
observed evolution in the luminosity-temperature relation. We quantitatively
investigate this departure from scaling laws. Within the local sample we used,
a moderate but clear variation of the amplitude of the gas mass fraction with
temperature is found, a trend that weakens in the outer regions. These
variations do not explain departure from scaling laws of our distant clusters.
An important implication of our results is that the gas fraction evolution, a
test of the cosmological parameters, can lead to biased values when applied at
radii smaller than the virial radius. From our $XMM$ clusters, the apparent gas
fraction at the virial radius is consistent with a non-evolving universal value
in a high matter density model and not with a concordance.
