A gaseous fuel mixer assembly for an engine includes a mixer housing forming gas delivery openings, and positioned to extend across a flow path formed by an intake conduit for the engine. A spool valve is within a central bore in the mixer housing and includes gas distribution openings selectively connectable to the gas delivery openings by moving the spool valve within the mixer housing using a piezoelectric actuator coupled with the spool valve by way of a pivot arm. Sealing lands of the spool valve are in an alternating arrangement with the gas distribution openings, such that at the closed position the sealing lands block the gas distribution openings from the gas delivery openings, and at the open position the respective openings are fluidly connected.