This paper proposes a power supply circuit for gate drive units (GDUs) that uses a one-turn transformer, which gives advantages in terms of cost and loss reduction. The structure of the proposed one-turn transformer consists of a primary winding in which the number of turns is one and a secondary winding in which there are multiple number of turns. The proposed one-turn transformer is connected in series with a switching device of the main circuit in order to obtain power for the GDUs. The proposed power supply circuit can be applied to all types of main circuit topologies such as a multilevel converter topology or matrix converters, in addition to a conventional six-arm inverter. In this paper, the design method of the one-turn transformer and its characteristics are described based on an equivalent circuit and fundamental experimental results with a step-down chopper. Besides, the proposed power supply circuit for GDUs is tested in a two-level three-phase inverter. Secondly, the obtained power for GDUs from the proposed one-turn transformer with three connection points is investigated by harmonics analysis from the viewpoint of core size and other parameters. Finally, experimental results confirm that the GDU in the two-level three-phase inverter with switching frequencies of 12.5 kHz and 16 kHz is operated by the proposed self-supplying gate power supply circuit without an external power source.