GD is optimum clean speed, not lowest...
The whole purpose of managed mode is that it manages the profiles and the speeds to be optimum. For example, if you change the cost index, all the speeds will change.
Now, if you fly an approach completely managed (which is almost impossible because of ATC restrictions), the aircraft will fly a continuous descent up to the 'D'-point, where it will automatically activate the approach fase and start decelerating towards final approach speed if you select flaps, which you better do or you will end up hot and high.
If we don't follow the profile because of radar vectors, and we activate the approach fase early, the managed mode will come back to an OPTIMUM speed, which happens to be minimum drag speed (same happens in the holding pattern) and thus GD. There is absolutely no reason for a managed speed to go below the green dot.
We as pilots on the other hand might have reasons to go below green dot, like ATC requests on extended downwinds. And although it is below the OPTIMUM (GD), it is still better then the alternative, which is extending the slats.
MR8