  We propose the Gauss-Bonnet dark energy model inspired by string/M-theory
where standard gravity with scalar contains additional scalar-dependent
coupling with Gauss-Bonnet invariant. It is demonstrated that effective phantom
(or quintessence) phase of late universe may occur in the presence of such term
when the scalar is phantom or for non-zero potential (for canonical scalar).
However, with the increase of the curvature the GB term may become dominant so
that phantom phase is transient and $w=-1$ barrier may be passed. Hence, the
current acceleration of the universe may be caused by mixture of scalar phantom
and (or) potential/stringy effects. It is remarkable that scalar-Gauss-Bonnet
coupling acts against the Big Rip occurence in phantom cosmology.
