The compound GdPtBi is known as a material where the nontrivial 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 was previously reliably determined, the magnetic interactions responsible for this type of order have 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 multidomain state of the k = (1/2 1/2 1/2) fcc antiferromagnet in the absence of a symmetry-breaking magnetic field. The magnon dispersion reaches an energy of similar to 1.1 meV and features a gap of similar to 0.15 meV. Using linear spin-wave theory, we determine 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 q phase and is dominated by second-neighbor interactions, thus featuring low frustration.