The catalytic ability of gold nanoparticles (AuNPs) toward the formation of H2 in the electrocatalyzed Hydrogen Evolution Reaction (HER) is thoroughly studied, using screen-printed carbon electrodes (SPCEs) as electrotransducers. The AuNPs on the surface of the SPCE, provide free electroactive sites to the protons present in the acidic medium that are catalytically reduced to hydrogen by applying an adequate potential, with a resulting increment in the reaction rate of the HER measured here by the generated catalytic current. This catalytic current is related with the concentration of AuNPs in the sample and allows their quantification. Finally, this electrocatalytic method is applied for the first time, in the detection of AuNPs as labels in a magnetoimmunosandwich assay using SPCEs as electrotransducers, allowing the determination of human IgG at levels of 1 ng/mL.