A head-mounted device may have a head-mounted support structure. Gaze tracking systems may be supported by the support structure so that the gaze of a user may be monitored. Lenses may be supported by the support structure. Display systems may provide computer-generated images to the user while the user is viewing real-world objects through the lenses. The gaze tracking systems may include image-sensor-based systems such as glint-based systems. A glint-based gaze tracking system may include light-emitting devices that emit light beams that create eye glints on the surface of a user's eyes and may include an image sensor that measures the eye glints to gather information on the user's gaze. A low-power gaze tracking system may be included in the head-mounted device. The low-power gaze tracking system may use light detectors to measure the magnitudes of respective light reflections of the light beams.