  A new diagnostic method, $Om$ is applied to generalized Chaplygin gas (GCG)
model as the unification of dark matter and dark energy. On the basis of the
recently observed data: the Union supernovae, the observational Hubble data,
the SDSS baryon acoustic peak and the five-year WMAP shift parameter, we show
the discriminations between GCG and $\Lambda$CDM model. Furthermore, it is
calculated that the current equation of state of dark energy $w_{0de}=-0.964$
according to GCG model.
