The fabrication procedure of efficient surface enhanced Raman scattering (SERS) platforms is demonstrated based on reproducibly photo-etched GaN epitaxial layers covered by a thin Au-Ag layer and subjected to partial de-alloying. Using a gold-rich Au57Ag43 alloy (70/30 wt. %), it is possible to preserve about 19 at. % of silver in the layer, even after etching in nitric acid for up to 24 h. A large enhancement of the Raman signal from the test 4-mercaptobenzoic acid molecules attached to such porous Au-Ag metal layer is obtained due to the presence of a high percentage of Ag. For the optimal etching time, in the range of 6–24 h, an enhancement factor (EF) greater than 107 was obtained. The contribution of different features of our SERS platforms such as the size of pores, nano-roughness, and the chemical composition of the metal layer for the EF parameter are critically discussed.