A new gold nanoparticles (AuNPs) based detection strategy using Electrochemical Impedance Spectroscopy (EIS) through Hydrogen Evolution Reaction (HER) is proposed. This EIS-HER method is used as an alternative to the conventional EIS based on [Fe(CN)6]3−/4− or [Ru(NH3)6]3+/2+ indicators. The proposed method is based on the HER induced by AuNPs. EIS measurements for different amounts of AuNP are registered and the charge transfer resistance (Rct) was found to correlate and be useful for their quantification. Moreover the effect of AuNP size on electrical properties of AuNPs for HER using this sensitive technique has been investigated. Different EIS-HER signals generated in presence of AuNPs of different sizes (2, 5, 10, 15, 20, 50 nm) are observed, being the corresponding phenomena extendible to other nanoparticles and related catalytic reactions. This EIS-HER sensing technology is applied to a magneto immunosandwich assay for the detection of a model protein (IgG) achieving improvements of the analytical performance in terms of a wide linear range (2–500 ng mL−1) with a good limit of detection (LOD) of 0.31 ng mL−1 and high sensitivity. Moreover, with this methodology a reduction of one order of magnitude in the LOD for IgG detection, compared with chroamperometric technique normally used was achieved.