It could be that Naruto has a special bond with Tenten, perhaps they are friends and he feels obligated to protect her from Gai who might be acting in a way that Tenten doesn't like. For example, if Gai's training methods are too intense for Tenten and she's in distress, Naruto would protect her.
In the fanfiction, there could be a prophecy or a mission that Naruto has to protect Tenten from Gai. It might also be that Naruto sees something in Gai's eyes, like an uncontrollable urge or a dark side emerging, that makes him think Tenten is in danger. Naruto, being the hero type, will not stand by and let his friend be in harm's way, so he decides to protect Tenten from Gai.
Maybe Gai has some sort of misunderstanding about Tenten and is acting aggressively towards her, so Naruto steps in to protect.
Naruto could use his shadow clone jutsu to create distractions. While Gai is busy dealing with the clones, Naruto can quickly get Tenten to a safe place.
In the fanfiction, it could be that Gai has been poisoned or influenced by a dark force. His behavior becomes erratic and he targets Tenten for some unfounded reason. Naruto, being the hero he is, jumps in front of Tenten to shield her. He might then try to find a way to cure Gai while also keeping Tenten safe. He could call on his friends like Sakura for medical help and use his own ninja skills to fend off Gai's attacks.
Perhaps in the fanfiction, there's a misunderstanding. Guy Sensei thinks Tenten has stolen a valuable jutsu scroll or something important to him. Naruto, knowing Tenten is innocent, stands in front of her. He might try to reason with Guy Sensei first, using his charm and quick - thinking. If that doesn't work, he'll be ready to fight, using his Rasengan if necessary.
Well, without actually reading the 'gai and tenten lemon fanfiction', it's hard to say precisely. However, if it follows the common trends in fanfiction, it could be a story that explores the characters of Gai and Tenten more deeply, perhaps their friendship or some unspoken feelings in a rather creative and fan - inspired setting.
One reason could be a change of heart. Maybe Nagini realizes that Voldemort's cause is wrong and sees something in Harry that makes her want to protect him, like his kindness or his will to fight for good.
One possible reason could be that Naruto feels betrayed by Konoha. Maybe he discovers some dark secrets about the village that make him question whether it's worth protecting. For example, if he finds out that the village elders were involved in unethical experiments or unjust actions against certain groups. Another reason might be that Naruto has different ideals now. He could believe that there are more important things in the world than just protecting Konoha, like bringing true peace to all nations without focusing on one village's interests.
His character development could be evident in his ability to empathize. Naruto has always been about protecting his friends, but in this case, he really tries to understand Tenten's fear and Gai's possible motives. He doesn't just use brute force. He might talk to Tenten, reassuring her, and then approach Gai with a more diplomatic stance at first. This shows that he has developed his emotional intelligence over time in the Naruto world.
One possible plotline could be that Naruto helps Tenten discover a new type of weapon or weapon - related jutsu. Since Naruto has a knack for finding new ways to do things, he might stumble upon an ancient weapon or an old scroll about weapons that he then shares with Tenten. This could set her on a new path of exploration and power - up in the fanfiction.
Sure, it's possible. Naruto has his own set of powers and a strong sense of justice. In the fanfiction world, he could see Tenten in trouble and decide to act. He might not fully understand the concept of gay fanfiction, but he doesn't need to. All he needs to do is recognize that Tenten is in a bad situation and use his abilities like the Rasengan or his ability to make friends and allies to get her out of it. It would be like one of his normal rescue missions, just in a different setting.
Another possibility is that he is in love with her. In fanfiction, love can be a powerful motivator. He might be attracted to her intelligence, her strong will, and her beauty. This love could override his loyalty to the Dark Lord and make him want to protect her at all costs. He could see her as a symbol of hope and a better future for the wizarding world.