15mm axles can be identified by the axle end cap configuration. Hubs equipped with 15mm axles will allow the user to simply pull off the freehub body without tools for easy maintenance or to swap freehubs.
Older model 12mm axles that require a 5mm allen key and a 17mm cone wrench to unscrew the axle end cap for service are not compatible with the 6-Pawl freehub.
The PowerTap 11 speed alloy freehub body is NOT compatible with the PowerTap GS Road hub. A DT Swiss 11 Speed Freehub Body will work with your PowerTap GS hub.