One possible reason could be for some sort of experiment. Gaster is often depicted as a mysterious scientist in the Undertale lore, and Sans has unique abilities. Maybe Gaster wants to study Sans' power over time and space. Another reason could be related to secrets. Sans might know something about Gaster's past or the true nature of the Underground that Gaster wants to keep hidden, so he kidnaps Sans to prevent him from revealing it.
In some fanfictions, sans might be made to kidnap for various reasons. One could be that he's misinterpreted a situation. Maybe he thinks the kidnapped person has some crucial information that could save his world or his friends. Another reason could be that he's been corrupted in some way, perhaps by an external evil force that's manipulating his actions.
One reason could be that Snape believes Hermione has knowledge that is dangerous if it falls into the wrong hands. So, he kidnaps her to keep her safe until he can figure out what to do with that knowledge. Another possibility is that he is being controlled by some dark magic or an outside force which compels him to take her against his will.
In some fanfiction, it could be due to a prophecy. Percy might think that by kidnapping Annabeth, he is fulfilling a part of a prophecy that would ultimately lead to a greater good. He could be trying to keep her away from a certain situation or enemy that the prophecy foretold. However, his way of handling it, which is kidnapping, is a very extreme measure.
One common theme is mystery. Since both characters have elements of mystery in their original lore, fanfictions often explore that further. Another theme is power dynamics. Given their different abilities, how they interact in terms of power is often explored. And friendship or a deeper connection is also a common theme as writers like to explore the relationship between these two unique characters.
One common theme is mystery. Since both Gaster and Sans have mysterious backstories in the original game, fanfictions often explore and expand on these mysteries, especially in relation to their relationship.
Sometimes, it could be a case of a mentally ill person. They might kidnap the girl on an impulse and gag her out of fear that she would escape or attract attention. They may not have a well - thought - out motive like ransom or revenge, but their actions are still very dangerous.
One reason could be revenge. Draco might feel that Harry has wronged him in some way throughout their years at Hogwarts, like getting him in trouble or making him look bad. So, he decides to kidnap Harry as a form of payback. Another reason is that Draco may be under the influence of his family. The Malfoys might see an opportunity to use Harry for their own gain, and Draco is just following their orders.
A sense of otherworldliness is common too. Since Gaster Sans is from a rather strange and unique universe, the fanfictions often play with the idea of the reader being in a very different, almost alien world. This can lead to themes of adjustment, as the reader tries to fit into this new world while being in a relationship with Gaster Sans.
Well, it could be about Gaster suddenly reappearing in the lives of Sans and Papyrus. Maybe he has some important knowledge or a mission for them. For example, he might need their help to fix something in the Underground that's going wrong.
Well, I'm not sure about all of them, but 'A Glimpse Beyond' is a sans x gaster fanfiction that's quite good. It shows their relationship in a more mysterious light, with a lot of plot twists. And then there's 'Fractured Souls' where they deal with some sort of crisis together, and it really shows the depth of their bond. Oh, and 'Whispers of the Ancients' is also worth checking out as it gives a different perspective on their characters and how they fit together in the story.