  We have performed a systematic magneto-optical Kerr spectroscopy study of
GaMnAs with varying Mn densities as a function of temperature, magnetic field,
and photon energy. Unlike previous studies, the magnetization easy axis was
perpendicular to the sample surface, allowing us to take remanent polar Kerr
spectra in the absence of an external magnetic field. The remanent Kerr angle
strongly depended on the photon energy, exhibiting a large positive peak at
$\sim1.7$ eV. This peak increased in intensity and blue-shifted with Mn doping
and further blue-shifted with annealing. Using a 30-band ${\bf k\cdot p}$ model
with antiferromagnetic $s,p$-$d$ exchange interaction, we calculated the
dielectric tensor of GaMnAs in the interband transition region, assuming that
our samples are in the metallic regime and the impurity band has merged with
the valence band. We successfully reproduced the observed spectra without
\emph{ad hoc} introduction of the optical transitions originated from impurity
states in the band gap. These results lead us to conclude that above-bandgap
magneto-optical Kerr rotation in ferromagnetic GaMnAs is predominantly
determined by interband transitions between the conduction and valence bands.
