If I'm understanding the question correctly, it's largely academic and answered in gameplay. Let's take an example:
Character A acts on DEX 18 during Phase 4. He chooses to use an Attack Power with the Gestures Limitation (specifics don't really matter here). This presumes that the character is able to perform the Gestures (i.e. not Grabbed, Entangled, or otherwise restrained). Just like any other Attack Power, it's going to work unless another character interrupts them. Without the Gestures Limitation, the options to interrupt Character A and prevent them from using the Attack Power are somewhat limited. With the Gestures Limitation, another character could have Character A Covered and use an Entangle or other restraint -- if they win the DEX roll-off (so that their Action goes before Character A's) and hit with the Entangle or other restraint, the Gestures automatically fail and the Power cannot be used.
For a non-Attack Power (that activates as a Zero Phase Action) all the same logic applies -- the activation itself can be stopped if another character acts before our intrepid hero...or the Power can be deactivated at any time if Constant (e.g. a force field) and Gestures are required throughout.