GSB is Sub-base course and mostly not considered in the main crust of the designed section of road. The prime work of GSB is drainage system and secondary to support the load transfer to the sub-grade.
WMM is a part of designed road section, thickness arrived from design calculations to withstand design load and ultimate transfer to sub0grade. WMM is mixed with designed proportion of aggregates and water content, laid and compacted in layers till designed thickness achieved.
The maximum aggregate size is limited to 65mm in GSB and WMM is limited to 20mm.