SNe Ia data in the framework of XCDM (X-matter with CDM), the hint for $w_X<-1$ has been found. Indeed, a study of high-$z$ SNe Ia [@Knop] finds that the equation of state of dark energy has a $99\%$ probability of being $<-1$ if no priors are placed on $\Omega_m^0$. When these SNe results are combined with CMB and 2dFGRS the $95\%$ confidence limits on an unevolving equation of state are $-1.46<w_X<-0.78$ [@Knop; @Riess] which is consistent with estimates made by other groups [@wmap; @sdss]. The possibility of $w_X<-1$ has provoked lots of investigations on the phantom dark energy [@phantom]. The remarkable feature of the phantom model is that the Universe will end its life with a “Big Rip” (future singularity) within a finite time [@bigrip]. On the other hand, we concern here another interesting proposal on dark energy, i.e. the dark energy component might be explained by a background fluid with an exotic equation of state, the generalized Chaplygin gas model [@chaplygin]. The striking feature of this model is that it allows for a unification of dark energy and dark matter. This point can be easily seen from the fact that the GCG behaves as a dust-like matter at early 