We investigate the transient surface emission and the optical gain of cubic GaN on GaAs(001) upon pulsed optical pumping at room temperature. The initial decay time of the transient surface emission drastically decreases with increasing excitation density, reaching a value as short as 20 ps at a fluence of 50 &mgr;J cm−2. This rapid decay suggests the presence of laterally amplified spontaneous emission. In fact, gain-stripe measurements of the edge emission reveal an optical gain exceeding 100cm−1at a fluence of 20 &mgr;J cm−2. ©1997 American Institute of Physics.