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.
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.
Tenten would likely be deeply traumatized. Naruto was not only a comrade but also a symbol of hope and strength. His death in protecting her would make her feel guilty. She might start to question her own worthiness of being saved at the cost of Naruto's life.
In the fanfiction, Naruto might be targeted by a powerful enemy who wants to steal his Nine - Tails chakra. Tenten, being a skilled ninja, steps in to protect him. She could set up all kinds of traps around their hiding place using her weaponry skills, and engage in combat with the enemy when they get too close, buying time for Naruto to regain his strength or for backup to arrive.
It could be rogue ninja from a hidden village. They might be after Tenten because she has some sort of valuable information or a powerful weapon in her possession. Naruto gets in their way to protect her, and these rogue ninja use their forbidden jutsus to overpower Naruto.
One possible plotline could be that Naruto and Tenten are on a mission together. Enemies attack and Naruto, being the selfless hero, puts himself in harm's way to protect Tenten. Maybe they are in a village that's being invaded, and Naruto uses his Rasengan to fend off multiple attackers while Tenten is injured or pinned down. In the end, Naruto sacrifices himself, leaving Tenten with a deep sense of loss and a newfound determination to carry on Naruto's will of protecting the village and its people.
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.
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 way this could unfold is that Naruto and Tenten are on a mission together. They are suddenly attacked by a powerful and unexpected enemy. Naruto, being the selfless hero he is, sees the enemy about to strike a fatal blow on Tenten. He jumps in front of her without hesitation, taking the full brunt of the attack. His last moments could be him telling Tenten to keep fighting and protect the village.