She might be initially shocked. Sakura has always seen Naruto as a friend, so learning of his romantic feelings could completely change her perspective of him. It could make her re - evaluate all their past interactions.
Another development could be that Sakura starts to develop feelings for Naruto as she spends more time thinking about his love for her. She might notice things about him that she never paid attention to before, like his kindness and determination in a more romantic way.
Some of his other friends might react differently too. Some may be more cautious around him, worried about the power of the jinchuuriki within him. But those who truly trust Naruto, like Shikamaru, might not change their attitude much. They would see that Naruto is still the same person he was before, just with this added aspect to his identity.
In many fanfictions, it might start with Sakura noticing small things about Naruto that she hadn't paid much attention to before. For example, his unwavering determination in the face of difficulties. This newfound awareness gradually grows into admiration and then love as she spends more time with him, either on missions or during downtime in the village.
Well, it depends on what you consider 'good'. There are fanfictions that explore the idea of Gaara and Sakura's love in a very sweet and innocent way, perhaps set in a post - war Konoha where they both help in the rebuilding process and slowly fall for each other. Others might take a more angsty route, with obstacles like their different responsibilities in their respective villages getting in the way of their love at first.
Well, it could be that Naruto is trapped in a genjutsu and is on the verge of losing his mind. Gaara, with his unique ability to sense Naruto's distress, breaks into the genjutsu world and saves Naruto. After that, they could have a deep conversation about their past and how they've both grown.
It could be Orochimaru. He's always been after Naruto for his special powers. He might set a trap for Naruto, but Gaara intervenes and saves him. Orochimaru's motives of immortality and his dark jutsu make him a great antagonist for this kind of story.
Well, in this kind of fanfic situation, Sakura could also become more protective of Naruto. Since a jinchuuriki is in a rather dangerous position, she might start to see him as someone who needs her support more than ever. She could try to keep his secret safe from those who might want to harm him because of it.