  The treatment of 1 + 3 covariant perturbation in a multifluid cosmology with
the consideration of f (G) gravity, G being the Gauss-Bonnet term, is done in
the present paper. We define a set of covariant and gauge-invariant variables
to describe density, velocity and entropy perturbations for both the total
matter and component fluids. We then use different techniques such as scalar
decomposition, harmonic decomposition, quasi-static approximation together with
the redshift transformation to get simplified perturbation equations for
analysis. We then discuss number of interesting applications like the case
where the universe is filled with a mixture of radiation and Gauss-Bonnet
fluids as well as dust with Gauss-Bonnet fluids for both short- and
long-wavelength limits. Considering polynomial f (G) model, we get numerical
solutions of energy density perturbations and show that they decay with
increase in redshift. This feature shows that under f (G) gravity, specifically
under the considered f (G) model, one expects that the formation of the
structure in the late Universe is enhanced.
