There is now (long overdue) expanded documentation of the user-facing features of GDC online on GCC's documentation site.
The GDC options documentation has been around since the start. What's new is the language reference section, covering code generation choices specific to GDC - what attributes are supported, intrinsics, pragmas, predefined versions, language extensions, missing features and deviations from spec, ...
More could be added or elaborated upon, such as what linkage do different symbols get, mixed language programming with C and C++, the anatomy of a TypeInfo and ModuleInfo object, and so on. This is enough as a first revision just to get it off the ground.
Being a first revision, expect some grammar mistakes, and unclear use of language in some parts throughout.
If any curious reader spots any such mistakes, or thinks there needs to be more clarity in some phrase or paragraph, you can either raise a bugzilla issue against GDC or name and shame me here.
Meanwhile GDC-13 development is still tracking DMD's mainline branch, no plans to switch to a stable D2 release just yet. DMD releases will resume a predictable cadence in the New Year, starting with the 2.102-beta release in January, meaning 2.102-final in February. So the expected D2 version GDC-13 would be using is v2.103.1 by the time our May release comes around.