A D-dimensional gravitational model with a Gauss-Bonnet term and the cosmological term is studied. We assume the metrics to be diagonal cosmological ones. For certain fine-tuned , we find a class of solutions with exponential time dependence of two scale factors, governed by two Hubble-like parameters H > 0 and h, corresponding to factor spaces of dimensions 3 and l > 2, respectively and D = 1 + 3 + l. The fine-tuned = (x, l, α) depends upon the ratio h/H = x, l and the ratio α = α 2 /α 1 of two constants (α 2 and α 1 ) of the model. For fixed , α and l > 2 the equation (x, l, α) = is equivalent to a polynomial equation of either fourth or third order and may be solved in radicals (the example l = 3 is presented). For certain restrictions on x we prove the stability of the solutions in a class of cosmological solutions with diagonal metrics. A subclass of solutions with small enough variation of the effective gravitational constant G is considered. It is shown that all solutions from this subclass are stable.