Luminescence experiments in magnetic fields up to 28 T on a freestanding gallium nitride (GaN) sample and a heteroepitaxial GaN layer lightly doped with silicon are presented. In these samples, the principal (DX)-X-0 recombination channel is accompanied by two electron satellites (TES), which involve the excited donor states as well as by the longitudinal-optic (LO) phonon replica of the principal transitions. When the internal donor excitations are magnetically tuned into resonance with the LO-phonon excitation, the intensity of TES is strongly enhanced and clear avoided-crossings between the LO-phonon replica of the principal (DX)-X-0 transition and TES involving highly excited states of oxygen and silicon are resolved. The observed behavior is explained in terms of resonant interaction between LO-phonons and donor-bound electrons. It is found that the resonant magnetopolaron interaction is stronger for the oxygen as compared to the silicon donor.