Plano-convex diffractive microlenses were fabricated in GaN-based materials with a gray-level mask for the first time. The surface relief of the gray-level diffractive microlens on photoresist was transferred onto GaN by inductively coupled plasma etching technique. The microlenses were characterized with a blue laser diode emitting at 405 nm. The focal length of the GaN diffractive microlens is 14.5 cm. The GaN microlenses can be used for the applications of the micro-optics systems in blue-violet-UV regions such as high-density optical data storage. The potential of the realization of the high-numerical-aperture diffractive microlens in GaN with gray-level mask is discussed.