Maybe your gripper is not attached by the time you enables the observer.
JavaDoc says;
Quote
Creates a new instance of the Frame Distance Condition. One of the frameA and frameB has to be attached to the robot flange at the time using the condition for a move command or when activating it in the ConditionObserver. During class initialization, it is accepted that neither of the frames are attached to a flange.
And Condition is mostly used with motions.
If you simply need an output regardless of main application, it might be easier for you to use cyclic backgroundTask with such a code like below.