Graphitic carbon nitrides form a popular family of materials, particularly as photoharvesters in photocatalytic water splitting cells. Recently, relatively ordered g-C3N4 and g-C6N9H3 were characterized by X-ray diffraction and their ability to photogenerate excitons was subsequently estimated using density functional theory. In this study, the ability of triazine-based g-C3N4 and g-C6N9H3 to photogenerate excitons was studied using self-consistent GW computations followed by solving the Bethe–Salpeter Equation (BSE). In particular, monolayers, bilayers and 3D-periodic systems were characterized. The predicted optical band gaps are in the order of 1 eV higher than the experimentally measured ones, which is explained by a combination of shortcomings in the adopted model, small defects in the experimentally obtained structures and the particular nature of the experimental determination of the band gap.