You'd have to go back to the HD discussion forum for a full history here, but as I understand it (others please correct me if I'm mistaken) here's the story. The "reference" monos are what eventually became the Xtreme monos - they were originally called "reference" because Klaus didn't know what to call them, there was a mini-naming contest here, and Klaus chose the Xtreme name for his new upgraded monoblocks. Therefore, the "reference" and the Xtremes are the same beast.
The "full Gary" was a special custom job that Klaus did for some guy (See the HD thread) that was really wiz-bang, and some of those upgrades resulted in the new Xtremes. The Xtremes do not have all the wiz-bang stuff of the "full Gary" version, but Klaus believed the cost of the "full Gary" would not result in a lot of sales for him (I think it was around 5 grand). So, if you've got the bux, give Klaus a call, and try to talk him into making you another "full Gary", if not, get the Xtremes, you'll love 'em.