The gain spectra of a submonolayer (SML) InGaAs/GaAs quantum dot (QD) laser working at 30℃ were measured using the Hakki–Paoli method. It is found that the maximum modal gain of QD ground states is as high as 44 cm−1 and no gain saturation occurs below the threshold at the lasing wavelength of 964.1 nm. When the injection current is about 0.98 times the threshold, the gain spectrum becomes symmetric with respect to the lasing wavelength, and zero-linewidth enhancement factor is observed. These properties are attributed to the high density and the high uniformity of SML QDs in our laser diode.