Magnetic coupling properties of Gd-doped ZnO nanowires are studied theoretically by using first-principles calculations. Several positions of Zn atoms that may be substituted by Gd atoms in ZnO nanowires are discussed. Numerical results show that the magnetic coupling is ferromagnetic when the two Gd atoms doped in ZnO nanowires are near each other. Injection of suitable amount of electrons can enforce these ferromagnetic properties in Gd-doped ZnO nanowires. It is also found that s-f coupling becomes remarkable when the Gd atoms are doped in ZnO nanowires, making the ferromagnetic coupling state more stable than the anti-ferromagnetic coupling state, and this is also the mechanism to elucidate the origination of ferromagnetic state in Gd-doped ZnO nanowires in experiments. These results will give a theoretical support for those who found experimentally that Gd-doped ZnO nanowires show ferromagnetic properties.