Another possibility is that Naruto gets angry. He has his own reasons for trying to help Sasuke, and being attacked by Sakura for it could make him frustrated. However, his anger would probably be mixed with confusion as he tries to understand what has gotten into Sakura.
He may also choose to defend himself without really fighting back. Naruto values Sakura as a friend and teammate, so he would be reluctant to harm her. He might just try to dodge her attacks and keep talking to her, hoping to snap her out of it.
In some fanfictions, Sakura's reaction could be more complex. She might be hurt not just physically but also emotionally. This could lead her to question her relationship with Sasuke and might cause her to make some big decisions, like whether to continue trying to reach out to him or to cut him off completely.
She could be heartbroken. Sakura has always loved Sasuke deeply, so his leaving for Naruto would be a huge blow to her. She might initially be in denial, not believing that Sasuke would choose Naruto over her.
Naruto might initially be in denial. He's so used to having Sakura around and believing that they have a future together. So when she leaves for Sasuke, he might think it's just a temporary thing or that she's been influenced by something or someone. But as time passes and he realizes it's real, he would be deeply hurt. He might also start to question his own self - worth, thinking that he wasn't good enough for Sakura. And he might even try to win her back at first, but then slowly come to accept the situation and focus on his other responsibilities like being a ninja and protecting the village.
One possible reason could be extreme jealousy. Sakura might be so blindly in love with Sasuke that she sees Naruto as a threat to Sasuke's attention or her relationship with Sasuke. Maybe she thinks Naruto is getting in the way of Sasuke's return to the village or her attempts to win Sasuke back.
Well, Naruto might be furious. He's always been protective of Sakura. He'd probably drop everything he's doing and go after Sasuke to rescue Sakura. Naruto has a strong sense of justice and loyalty, especially towards his friends. And Sakura is one of his closest friends, so he won't stand by and let her be in danger.
Naruto would be devastated. He's always been one to value his friends deeply. He might initially go into denial and then be filled with rage, vowing to find out who or what was responsible for her death and seek justice. He would also likely carry a great deal of guilt, thinking that he should have been able to prevent it.
There would be other fans who might be intrigued. They like to explore different 'what - if' scenarios in fanfiction. They would see it as a creative way to explore the characters in a new light, even if it goes against the canon. However, they would also be aware that it's just a fictional alternative and not the real Naruto story.
Naruto, being the friendly and optimistic one, would likely be excited for Gaara. He might encourage Gaara to be more open about his feelings towards Sakura. He could offer advice based on his own experiences with Hinata. Sasuke, though, would probably be aloof. He has always been more focused on his own goals and revenge. He might view Gaara's crush as a distraction for Gaara and tell him to focus on more important things, like protecting his village. However, deep down, he might also hope that Gaara can find some happiness.
On the other hand, there would be fans who would welcome it. Maybe they never liked the pairing in the first place or they think it adds an interesting twist to the story. They could start exploring new relationships for both Sasuke and Sakura in their own fan - created stories.
Naruto might react with confusion at first. He has always wanted to be accepted by the villagers, so being attacked is the opposite of what he expected.