  Corrections to solar system gravity are derived for f(G) gravity theories, in
which a function of the Gauss-Bonnet curvature term is added to the
gravitational action. Their effects on Newton's law, as felt by the planets,
and on the frequency shift of signals from the Cassini spacecraft, are both
determined. Despite the fact that the Gauss-Bonnet term is quadratic in
curvature, the resulting constraints are substantial. It is shown that they
practically rule out f(G) as a natural explanation for the late-time
acceleration of the universe. Possible exceptions are when f(G) reduces to
something very close to a cosmological constant, or if the form of the function
f is exceptionally fine-tuned.
