Plasmonic catalysis is emerging as a dynamic field in heterogeneous catalysis and holds great promise for the efficient utilization of solar energy. Central to the development of plasmonic catalysis is the design of efficient plasmonic nanocatalysts. In this report, plasmonic gap nanostructures (PGNs) on the basis of Au@poly(o-phenylenediamine) (POPD)@Pd sandwich nanostructures are synthesized as plasmonic nanocatalysts by an in-situ reduction synthetic strategy, which allows for the precise engineering of the POPD gap size between plasmonic Au and catalytic Pd components. The introduction of conducting POPD nanogap in PGNs not only effectively enhances their light harvesting capability, but also provides an effective charge transfer channel for harnessing the photogenerated hot charge carriers. In this respect, distinct gap-dependent performances in plasmon-enhanced electrocatalysis of ethanol oxidation reactions (EOR) are demonstrated with the PGN nanocatalysts and over 2.5 folds of enhancement can be achieved. A volcano plot is derived to describe the relationship between the catalytic activities and gap size of the PGN nanocatalysts, which is well explained by the interplay of their light harvesting and charge transport capabilities. These results highlight the importance of gap engineering in PGNs for plasmonic catalysis and offer the promise of developing efficient plasmonic nanocatalysts for other heterogeneous catalytic reactions.