And when you open the gate? Sodium ions rush into the cell, essentially heading down two gradients at once.
That creates a drastic and rapid change in the membrane’s potential, called “depolarisation” — because the charges are becoming less polarised, or separated.
(It’s a little confusing to talk about the change in membrane potential. In fact any movement towards a “more positive” membrane potential is called “depolarisation”, even if the resulting potential is still negative. And any movement toward a more negative potential is called “hyperpolarisation”. This only makes sense because the resting potential is already negative.)