We discuss the key science drivers and the derived instrument requirements. GIRMOS is being designed from the ground up to be an AO survey instrument by maximizing its etendue and throughput for narrow field and multi-object observations, as well as minimizing acquisition and AO setup times. Design optimizations have been made to minimize overheads and calibrations, as well as reduce the impact of pointing drift for long exposures. As a result, GIRMOS will offer unparalleled survey efficiency by reaching up to ten times more survey throughput over existing single-object spectrographs. Finally, we also present the preliminary design of the instrument, particularly its architecture, overall optical design, and key subsystems. GIRMOS is being developed in close collaboration with the GNAO facility and will be commissioned as its first light instrument.