Zero! This is one of those things that math nerds like to drone on about for eons. (Pro tip - don’t invite me to a party.) Something can be perfectly GN everyplace except a single point.
It’s a little easier to think about with curves.
At a subd “star point” it is often the case that the the curvature simply does not exist. But it is alwasy the case that at every single point nearby but not equal to the star point, the SubD surface is G2.