A gain adjustment apparatus includes a first output device configured to output a first audio signal, a second output device configured to output a second audio signal, a memory, and a processor coupled to the memory and configured to convert the first audio signal and the second audio signal to a first frequency spectrum and a second frequency spectrum, calculate an estimated difference between the first frequency spectrum and the second frequency spectrum based on a comparison of the first frequency spectrum with the second frequency spectrum, and output a first and second adjustment spectra corresponding to the first and second frequency spectra, the first and second adjustment spectra being adjusted on the basis of the first and second frequency spectra and the estimated difference.