The doneness is set by your bath temperature. Doneness relates to the internal temperature of the finished product.
Time impacts not the doneness but the tenderness. Tenderness results from conversion of collagen to gelatin. This takes time at sous vide temperatures.
The gradient in your steak is most likely the result of your wearing process. This is when any overcooking us likely to occur because the steak is already cooked and care must be taken not to push the interior further.