Gap-closer refers to an ability to get to the point you are trying to reach faster, and closing the gap between the two points. The best known example of this is probably Blink, although other abilities such as Charge and Death Grip would apply as well. A speed buff would also be considered a gap closer because you are going from point a to b faster.

Gap-closers are particularly relevant in arena, as are other skill groups such as interrupts and dispels.