  The compound GdPtBi is known as a material where the non-trivial topology of
electronic bands interplays with an antiferromagnetic order, which leads to the
emergence of many interesting magnetotransport phenomena. Although the magnetic
structure of the compound has previously been reliably determined, the magnetic
interactions responsible for this type of order remained controversial. In the
present study, we employed time-of-flight inelastic neutron scattering to map
out the low-temperature spectrum of spin excitations in single-crystalline
GdPtBi. The observed spectra reveal two spectrally sharp dispersive spin-wave
modes, which reflects the multi-domain state of the $\mathbf{k} =
(\frac{1}{2}\frac{1}{2}\frac{1}{2})$ face-centred cubic antiferromagnet in the
absence of a symmetry-breaking magnetic field. The magnon dispersion reaches an
energy of $\sim 1.1$~meV and features a gap of $\sim 0.15$~meV. Using linear
spin-wave theory, we determined the main magnetic microscopic parameters of the
compound that provide good agreement between the simulated spectra and the
experimental data. We show that GdPtBi is well within the
($\frac{1}{2}\frac{1}{2}\frac{1}{2}$) phase and is dominated by second-neighbor
interactions, thus featuring low frustration.
