802.11 is a widely-used wireless communication standard today and is still under constant evolution. Two major enhancements of the standard, 802.11n and 802.11ac, boost the performance and quality of service, with the former getting support in nearly all commercial devices today and the latter gaining prevalence. However, there is currently no software-defined radio (SDR) system that is capable to support the whole 802.11ac protocol stack, especially the MAC layer, in real-time, limiting research and testing on these latest innovations. This paper presents GPLM, a Low-MAC architectural design for FPGA-based SDR systems that supports 802.11ac. We identify challenges imposed by new features in 802.11ac MAC layer, and make careful architectural choices to ensure both standard compliance and flexibility. The design and implementation of critical modules and the employment of software hardware co-design are detailed in this paper. Paired up with an existing work on the PHY layer implementation of 802.11ac, the implementation of GPLM is validated from various aspects.