Magneto-luminescence experiments performed on a thick, freestanding GaN layer in magnetic fields up to 28 T are presented. The photoluminescence spectrum of this material shows narrow emission lines due to neutral donor-bound excitons ((DX)-X-0). In addition to the principal recombination channel of (DX)-X-0 in which the donor is left in its ground state, two-electron satellites (TES) involving different excited donor states, as well as LO phonon replica of the principal and satellite transitions are observed. For the magnetic field range in which internal donor excitations are tuned into resonance with the LO phonon excitations, the intensity of TES is strongly enhanced. A clear resonant interaction between TES corresponding to the highly excited donor states (n > 4) and LO phonon replica of (DX)-X-0 is observed. The observed effect is discussed in terms of electron-phonon interaction.