One common theme is friendship. Nate and Karissa often support each other through thick and thin in many fan fictions. Another theme is growth. They might grow as individuals or as a pair, learning new things about themselves and each other.
There's a fan fiction called 'Nate and Karissa: A New Beginning'. In this story, Nate and Karissa start a new chapter in a small town where they face various difficulties in fitting in but eventually find their place. It has really well - developed characters and an engaging plot. Also, 'The Bond between Nate and Karissa' is quite good. It delves deep into their relationship and how they overcome misunderstandings.
Big Nate leans towards realistic fiction. It shows common situations and emotions that kids go through at school, but it adds a touch of fun and imagination to keep it interesting.
Big Nate is definitely fiction. The characters, events, and settings are all created from the author's imagination to entertain readers and bring fun and excitement.
Perhaps it's a mistake and you actually meant 'gate' in a different sense like 'gateway' to detective fiction. In that case, Sherlock Holmes could be considered such a 'gate' as his popularity has led many people to become interested in detective stories. His character has been adapted numerous times in various forms of media, which has broadened the appeal of detective fiction.