The current policy for GC wrappers, as keithp explained it to me, is:

The contents of GC funcs/ops vectors (the structs) may not change after they are
in use, i.e. CreateGC.
GC funcs may change the funcs and ops vectors (the pointers) for the GC.
GC ops may change the ops vectors for the GC.

Because of this, the rule for GC wrapping is:

In a GC func, unwrap funcs and ops, call down, pull funcs and ops back out of
the GC (to maintain the wrapping chain if the call down resulted in
modification), and rewrap funcs and ops.
In a GC op, unwrap ops, call down, pull ops back out, and rewrap ops.

The documentation in section 5.8.2 of the DDX document seems both ambiguous and
not in line with this policy, and needs to be updated.