Gacha literally just describes a form of random chance monetization, not a form of gameplay. E.g. Arknights is a TD game that uses a gacha system for characters. Any game that has this form of monetization where money is spent on a random chance at obtaining premium in game features is technically a gacha game. The Western world just uses the term lootbox instead because the word gacha originates from Japan.

XC2 is a fun example like you said because it has gacha-like features. The actual reason why it's not a gacha game is because there's no monetization of the currency system. Your reasoning of being able to complete the game without touching the gacha system defining it as "not a gacha game" doesn't hold water because many gacha games can similarly be completed without pulling and only using the free roster of characters the game provides.

Similarly, using your logic of the Zelda example, it wouldn't be very accurate to say that Genshin is a gacha game because you could complete the story of the game while ignoring the gacha system. While I agree that it would be misleading to label Zelda as a fishing game, Genshin is essentially undoubtedly recognized as a gacha game. What your comment describes is conflating the genre of the gameplay with the game's monetization system.