  We measured the magneto-reflectivity spectra (4 - 90 meV, 0 - 16 T) of the
triple-point semimetal GdPtBi and found them to demonstrate two unusual broad
features emerging in field. The electronic bands of GdPtBi are expected to
experience large exchange-mediated shifts, which lends itself to a description
via effective Zeeman splittings with a large g factor. Based on this approach,
along with an ab initio band structure analysis, we propose a model Hamiltonian
that describes our observations well and allows us to estimate the effective g
factor, g* = 95. We conclude that we directly observe the exchange-induced
$\Gamma_{8}$ band inversion in GdPtBi by means of infrared spectroscopy.
