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/ρα gCg, 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,MNRAS, 334, L11; 2003, 342, 257) and the dimensionless coordinate distances to type Ia supernovae and Fanaro Riley type IIb radio galaxies compiled by Daly & Djorgovski (2003, ApJ, 597, 9). We obtain the confidence region on the two parameters fully characterizing gCg, As ≡ A/ρ(1+α) gCg0 and α, from a combined analysis of these databases, where ρgCg0 is the energy density of gCg at present. It is found that As=0.70+0.16−0.17 and −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 α=0. Particularly, the standard Chaplygin gas (α=1) is ruled out as a feasible UDME by the data at a 99% confidence level.