Carbon doping of GaN is of great interest in part because it has been shown to result in semi-insulating (SI) behavior. However, determination of the bandgap states and hence the exact mechanism responsible for the SI behavior is, to date, an unresolved issue. A key impediment is that the presumed C acceptor levels are likely near the minority carrier (valence) bandedge of otherwise background n-type GaN, and hence their precise detection by usual methods is difficult. In this paper, we exploit the inherent ability of deep level optical spectroscopy (DLOS) to detect states near the minority carrier band edge, as well as potentially deep states associated with C in background n-type GaN. This is accomplished by comparing unintentionally doped (uid) GaN grown by atmospheric pressure (AP) MOCVD, which has residual n-type conductivity, with LP MOCVD GaN that incorporates a large concentration of C for both uid and intentionally Si co-doped conditions. The results show the emergence of a shallow state at E
- 3.28 eV (E
+ 0.16 eV) for the LP samples with a minimum concentration of 3.6 × 1016 cm-3 that efficiently compensates Si donors for the co-doped sample, and results in semi-insulating behavior for the uid-LP sample. In contrast, this state is not observed for the AP GaN material, which incorporates a factor of ∼10 times less C, and instead only the expected residual Mg acceptor level at E
- 3.22 eV is observed. Additionally, a state at E
- 1.35 eV, near the theoretically expected C split-interstitial level in n-type GaN, is observed to increase significantly in concentration with increased C concentration.