Just noticed this forun. I sent an e-mail to Garmin back in early January, asking about the ggz file format. The reply was:
The GGZ file format will simply be another supported file format that the Oregon 600 can use. The device will continue to use the GPX file format as well.
It didn't answer much, but we can see it will still use the gpx format. To bad there isn't more info available.