In order to estimate and compare the would‐be performances of both hexagonal and cubic GaN quantum‐well lasers, the optical gains of strained‐layer hexagonal and cubic GaN quantum wells are studied theoretically taking into account non‐Markovian relaxation. The 6×6 multiband effective‐mass Hamiltonians are used to calculate the band structures of both hexagonal—and cubic—quantum wells. As a numerical example, the optical gains of strained cubic and hexagonal quantum wells with various well widths are calculated. It is expected that the optical gains of the cubic‐phase quantum well are larger in magnitudes than those of the hexagonal GaN quantum well over the wide range of carrier densities due to the heavier effective mass of the HH1 band of the latter at the zone center. ©1996 American Institute of Physics.