Distributed maximum power point tracking (MPPT) photovoltaic (PV) systems have drawn increased attention since the inhomogeneous irradiance on PV modules can significantly deteriorate the effectiveness of solar energy harvest. To address the issue, this paper focuses on developing a highly efficient submodule integrated converter (subMIC), which is connected across each submodule of a PV panel. Gallium nitride (GaN) field-effect transistors are utilized in the synchronous buck converter topology to achieve high conversion efficiency and reduced size. A converter prototype, which is digitally controlled using a microcontroller for MPPT, is developed and tested, considering various irradiation conditions. The peak efficiency of the proposed GaN-based subMIC is measured to be higher than 99% at 400-kHz switching frequency, whereas the accuracy of MPPT is above 99% considering low-power conditions.