We model the cosmic medium as the mixture of a generalized
Chaplygin gas and a pressureless matter component. Within a
neo-Newtonian approach we compute the matter power spectrum. The
2dFGRS data are used to discriminate between unified models of the
dark sector and different models, for which there is separate dark
matter, in addition to that accounted for by the generalized
Chaplygin gas. Leaving the corresponding density parameters free,
we find that the unified models are strongly disfavored. On the
other hand, using unified model priors, the observational data are
also well described, in particular for small and large values of
the generalized Chaplygin gas parameter α.