Photoluminescence (PL) and optically-detected magnetic resonance (ODMR) experiments have been performed on undoped GaN epitaxial layers grown on 6H-SiC substrates. The defects observed in these films are compared with those found from previous ODMR studies of undoped GaN layers grown on sapphire substrates. Strong, sharp donor-bound exciton bands at 3.46 -3.47 eV and weak, broad emission bands at 2.2 eV were observed from several 0.7 and 2.6 μm-thick films. In addition, fairly strong shallow donor - shallow acceptor (SD-SA) recombination with a zero-phonon-line at 3.27 eV was found for GaN layers less than 1 μm-thick. The first observation of magnetic resonance on this SD-SA recombination from undoped GaN is reported in this work. Two magnetic resonance features attributed to effective-mass (EM) and deep-donor (DD) states were detected on the 2.2 eV emission bands from all the GaN/6H-SiC films. These resonances were observed previously on similar emission from undoped GaN layers grown on sapphire substrates. The same EM donor resonance, though much weaker, was also found on the SD-SA recombination. However, a resonance associated with shallow acceptor states was not observed on this emission. The weakness of the donor resonance arises from the weak spin-dependence of the recombination mechanism involving spin-thermalized shallow acceptors. The absence of an EM acceptor is due to the broadening of the resonance through the spreading of the acceptor g-values by random strains in these films.