I have a Gemini with a Thru 12 hub. AFAIK, any hub with standard spacing (135mm ??) and the ability to take a 12mm diameter axle should fit - however, you will need a specific Cannondale axle for the hub. The non-drive side of the swingarm has a slightly ovalised hole to accept the ovalised axle. I'm not sure whether these axles can be bought separately....
There's an owners manual supplement for Gemini Thru 12 available as a pdf from www.cannondale.com (see tech section)
There's an owners manual supplement for Gemini Thru 12 available as a pdf from www.cannondale.com (see tech section)