  The current work treats cosmological perturbation in a mixture of standard
matter, Chaplygin gas as well as Gauss-bonnet fluids using a 1+3 covariant
approach in the context of modified $f(G)$ gravity. We define the gradient
variables to obtain linear perturbation equations. After scalar and redshift
transformations, we consider both an original Chaplygin and generalized
Chaplygin gas models under Gauss-bonnet gravity. For pedagogical purposes, the
consideration of polynomial $f(G)$ gravity model was used to solve the
perturbation equations for short- and long- wavelength modes and investigate
the late time evolution. The numerical solutions were obtained. The results
show that the energy overdensity perturbations decay with an increase in
redshift. The treatment recovers GR results under limiting cases.
