VorbisGain is not part of the decoder spec and will probably never be
in its current form because the main Vorbis devs don't approve of the way it's currently handled. They want Vorbis Comments to be used for human-readable data only; to support Replay Gain the way they would like it (i.e., to store the gain information in the file header) would, AFAIK, require a change to the structure of the Vorbis header or something like that.
Remember, VorbisGain is actually more or less a "hack". That's not to say one shouldn't use it though