  Although various cosmological observations congruously suggest that our
universe is dominated by two dark components, the cold dark matter without
pressure and the dark energy with negative pressure, the nature and origin of
these components is yet unknow. The generalized Chaplygin gas (gCg),
parametrized by an equation of state, $p = -A/\rho_{\rm gCg}^{\alpha}$, was
recently proposed to be a candidate of the unified dark matter/energy (UDME)
scenarios. In this work, we investigate some observational constraints on it.
We mainly focus our attention on the constraints from recent measurements of
the X-ray gas mass fractions in clusters of galaxies published by Allen et al.
(2002,2003) and the dimensionless coordinate distances to type Ia supernovae
and Fanaroff-Riley type IIb radio galaxies compiled by Daly and Djorgovski
(2003). We obtain the confidence region on the two parameters fully
characterizing gCg, $A_s \equiv A/\rho_{{\rm gCg}0}^{(1+\alpha)}$ and $\alpha$,
from a combined analysis of these databases, where $\rho_{{\rm gCg}0}$ is the
energy density of gCg at present. It is found that $A_s=0.70^{+0.16}_{-0.17}$
and $\alpha=-0.09^{+0.54}_{-0.33}$, at a 95% confidence level, which is
consistent within the errors with the standard dark matter + dark energy model,
i.e., the case of $\alpha = 0$. Particularly, the standard Chaplygin gas
($\alpha=1$) is ruled out as a feasible UDME by the data at a 99% confidence
level.
