An Optical Network Unit (ONU) emulator enables traffic emulation in a Gigabit-capable Passive Optical Network (GPON) that couples an Optical Line Termination (OLT) system to the ONU emulator and carries mixed types of emulated ONU traffic and real ONU traffic. The ONU emulator includes one or more emulated ONU platforms; each emulated ONU platform includes multiple ONU emulator blocks, and each ONU emulator block includes circuitry to emulate multiple emulated ONUs simultaneously. An emulated ONU platform generates upstream GPON frames according to respective profiles of the emulated ONUs, and sends the upstream GPON frames to the OLT system. At least a subset of the upstream GPON frames reach the OLT system via one or more passive optical splitters that couple the emulated ONU platform and a set of real ONUs to the OLT system. The emulated ONU platform also receives downstream GPON frames from the OLT system.