This Project aims at the development of a contactless ammeter based on GMI sensors for measuring leak currents in surge arresters. The first step consisted on analyzing typical characterization curves of the impedance phase as a function of external magnetic field variation, previously measured for GMI samples, as the basis for the computational model. Next, an excitation and polarization circuit, previously developed in the Master s Dissertation in Metrology of Pedro Alejandro Duarte Riveros, was adapted in order to generate an output voltage directly proportional to the intensity of the leak current. A detailed explanation about each circuit module s functioning is presented throughout the text, as well as the computational simulation results, carried out in TOPSPICE program, containing relevant comments on the understanding of this results. Next, the resolution of the developed magnetic transducer is estimated from spectral noise density data, and so compared with the required resolution for a reliable measuring of typical leak currents in surge arresters. The results prove the possibility of using the GMI magnetic sensor, as long as it is implemented in the gradiometric configuration, generating considerably satisfactory responses for the purpose for which it was designed.