The GAP Design elevates the feed point.
Radiation resistance increases as the feed point is elevated.
If the proper elevation is selected, the radiation resistance will be 50 ohms - a perfect match to the feedline. Elevating the feed point also virtually eliminated the earth loss. You don't find loss with dipoles, because they are elevated. Technically, earth loss results from the capacitance of the antenna to ground, from above the feed point. That is the key.