Perhaps Gale is being manipulated by some other force. There could be a new group or individual in the story that has convinced Gale that kidnapping Katniss is the only way to achieve a greater good, like starting a revolution. They might have fed him false information about Katniss being the key to a successful uprising.
The Capitol's propaganda machine could be the main antagonist. It could have spread so much misinformation that Gale believes he has no other choice but to kidnap Katniss. The Capitol's influence might have distorted Gale's view of the situation, making him think that this extreme action is necessary to protect Katniss from a false threat that the Capitol has created.
One possible plotline could be that Cato kidnaps Katniss as a strategic move in the Hunger Games. Maybe he thinks he can use her to draw out other tributes or get an advantage over the Careers. Another could be that Cato has some sort of personal vendetta against Katniss that leads him to kidnap her outside of the normal rules of the Games.
Well, it could be that Gale has just returned from a dangerous mission, and Katniss is so relieved to see him alive that they kiss. Or perhaps they're hiding from the Capitol's soldiers in a small, intimate space, and the close quarters lead to a kiss. There could also be a scenario where they're reminiscing about their past and all the unspoken feelings come to the surface, resulting in a kiss.
Perhaps Gale decides to hide his feelings at first. He could watch in silence, his face showing a range of emotions from shock to sadness. Later, he might try to distance himself from Katniss, realizing that she has strong feelings for Peeta.
One possible plot could be that Lotor kidnaps Lance to use him as a bargaining chip against the Paladins. He might try to force Lance's friends to give up something valuable in exchange for his safe return.
One possible plot could be Clary using her shadowhunter skills to try and escape from Jonathan's clutches. Maybe she finds a hidden weapon or a secret passage in the place where she's held captive.
One possible plot could be that the hunter is a villain from Spiderman's rogues gallery who mistakes a die - hard Spiderman fan for someone important to Spiderman. Maybe he wants to use the fan as bait to lure Spiderman into a trap. Another plot could involve the hunter being from a different dimension or world and thinking the fan has some sort of power related to Spiderman, so he kidnaps the fan to study or harness that power.
Their relationship would likely become more intense. With a baby on the way, they'd have to rely on each other more. Gale might become more protective of Katniss, and Katniss would have to trust Gale in new ways.
In some fanfictions, Gale and Katniss might go on a hunting trip together like they used to before the Games. But this time, they get lost in the woods and have to rely on each other more than ever. This shared experience brings them closer and they end up confessing their love for one another. During their time lost, they talk about their hopes and dreams for the future, away from the Capitol's control.
Another theme is the struggle with their different worldviews. Gale is more radical in his approach to the Capitol, while Katniss has her own way of dealing with things. Fanfictions often play with this contrast.