The Gitzo GS5000C Disk Adapter allows users of the current generation of Gitzo tripods (featuring the Ground Level Set, with the column's spring hook affixed by a M10 thread) to mount an additional disc or other various tripod accessories in place of the normal spring-loaded hook assembly.  This adapter isn't necessary nor compatible with the previous generation of tripods without the Ground Level Set; they already had a 3/8"-16 thread at the bottom of the column.

With this adapter in place, you can attach the tripod's "Power Disc" top plate to the bottom of the column, and of course use a tripod head on the disc