GaMnN and Mg-codoped GaMnN films were grown in molecular beam epitaxy using a GaN single precursor. The structural, electrical, and magnetic properties were examined. The Mg-codoped GaMnN layers revealed room-temperature ferromagnetism. Codoping with Mg reduces the Mn incorporation, but increases the conductivity of the GaMnN films. At the same time, the saturation magnetization and coercivity have increased at room temperature. As this improvement was obtained with much reduced Mn concentration of ∼0.3%, the crucial role of the carriers for carrier-induced ferromagnetism in magnetic semiconductors is observed.