The use of gaseous fuels in internal combustion engines is increasing, due to several reasons, first of all their low environmental impact, large availability and low cost. Nevertheless, the need to reduce emissions also from gas engines is an important aspect to be considered in order to comply with future engine emissions regulations. In this scenario, an extensive experimental activity was performed to fully characterize an heavy duty spark ignition engine, under development for Euro VI compliance and designed to run with gaseous fuels. Two separate sets of experiments were carried out, in order to analyze the engine behavior when burning LPG and CNG, respectively. To this aim, the engine was installed on a dynamic test bench, accurately instrumented to characterize the combustion evolution, performance and exhaust pollutant emissions, along the World Harmonized Transient Cycle (WHTC), the new European driving homologation cycle. The main part of the manuscript addresses the analysis of the exhaust particulate emissions, in terms of soot concentration, particle number (PN) and particle size distribution function (PSDF). More in detail, a photo-acoustic sensor and a fast particulate spectrometer were adopted for on-line soot, PN and particle size measurements, during the transient engine tests. The results revealed that although the gaseous emissions were within homologation limits, soot and PN could represent an issue for this class of engines. The experiments allowed to highlight that most part of the particles are emitted during specific phases of the driving cycle and could be ascribed to the engine oil vapors combustion. Moreover, the investigation, indicating which engine operating conditions displayed the highest contribution to particles emissions, may provide helpful insights to deal with such critical conditions.