  In this paper, we consider a gravitational action containing a combination of
the Ricci scalar, $R$, and the topological Gauss--Bonnet term, $G$.
Specifically, we study the cosmological features of a particular class of
modified gravity theories selected by symmetry considerations, namely the
$f(R,G)= R^n G^{1-n}$ model. In the context of a spatially flat, homogeneous
and isotropic background, we show that the currently observed acceleration of
the Universe can be addressed through geometry, hence avoiding \emph{de facto}
the shortcomings of the cosmological constant. We thus present a strategy to
numerically solve the Friedmann equations in presence of pressureless matter
and obtain the redshift behavior of the Hubble expansion rate. Then, to check
the viability of the model, we place constraints on the free parameters of the
theory by means of a Bayesian Monte Carlo method applied to late-time cosmic
observations. Our results show that the $f(R,G)$ model is capable of mimicking
the low-redshift behavior of the standard $\Lambda$CDM model, though
substantial differences emerge when going toward high redshifts, leading to the
absence of a standard matter-dominated epoch. Finally, we investigate the
energy conditions and show that, under suitable choices for the values of the
cosmographic parameters, they are all violated when considering the mean value
of $n$ obtained from our analysis, as occurs in the case of a dark fluid.
