For being gods, the Giants go down embarrassingly easily when confronted by the Gold Saints. That actually references the Greek myth: the Giants were all killed by either being wounded by a mortal (specifically Herakles. The prophecy actually stated any mortal could work, but Zeus and Athena summoned him because he was the strongest of them all) and finished off by a god or wounded by a god and finished off by a mortal... And the Saints, as mortals under the protection of Athena, count as both attacking at once.
This also explains why the Titans are genuinely surprised when that happens and they themselves are wounded by the Gold Saints: the means to kill gods became known after the Titans were imprisoned in the depths of Tartarus, as far they knew they only had to fear the power of the Olympians and renewed imprisonement.
The Gold Saints at first take the Giants seriously, with the exceptions of Shaka and Death Mask. While Shaka is comprehensible in light of his powers, the weakest of the Gold Saints is a surprise... Until one takes into account he's from Italy and was trained there, specifically Sicily: like most Italians he was immersed in Greek mythology just by being there and was trained right where the king of the Giants was imprisoned (Zeus threw Sicily (the whole island) at him), he simply knew that a Giant could be easily killed by the attacks of a god and a mortal in quick succession and, in a moment of this trope happening in-universe, realized a Gold Saint protected by Athena counted for both.
The tie-in novel "Gigantomachia" hints that Death Mask normally stays in Sicily specifically to keep guard over Typhon, the aforementioned king of the Giants. Of course he's versed on how to deal with the Giants, it's literally in his job description.
"Gigantomachia" also hints to why he doesn't take part in much of the fight: he's busy making sure Typhon stays sealed and doesn't either bring reinforcements to the enemy or becomes a third side in the war.
Pontus' last words to the Gold Saints about how they will become Athena's enemies foreshadows their role in the classic Sanctuary Arc. As Mu and Dohko are not there they would still follow Athena. Additionally, Deathmask and Aphrodite already betrayed her when they learned the truth about the Pope.
