Gildarts is a character who exists mostly off-page, which I think explains a lot about his role in major battles. He's so overwhelmingly powerful that if he were around for every crisis, there'd be no tension. The guild would just send him and the arc would be over in a chapter. So narratively, he's often conveniently missing or incapacitated, which is a common trope for OP mentors.
When he does show up, it's for specific purposes. Against Bluenote during the Tenrou Island arc, his fight wasn't really about winning; it was about demonstrating the sheer scale of power Natsu and the others needed to aspire to. He shattered the landscape just by clashing. That moment redefined 'S-Class' for the reader. Later, against Acnologia, his role was to be the first to deliver a meaningful, if ultimately futile, blow that gave everyone else a sliver of hope. He's less a participant and more a narrative benchmark.
His real impact is psychological. For Natsu, he's this impossible ideal. For Cana, he's a deeply personal conflict. That duality—the guild's unshakable pillar and a failed father—adds more texture to battles than his actual spells do. The fights are just the flashy exterior.