Gilgamesh´s fighting style completely counters Sasaki, and the narration stating how he fought off everyone doesn´t include "and some blonde douchebag". The very first shot would break his laundry pole in half if a strike from Saber can even partially bend it. Considering how powerful Gil is and that he has more mana than he could possibly use he probably just brute-forced his way through the Ryoudou Temple bounded field.