A great Buffy and Giles lemon fanfiction is 'Eternal Flame'. It takes their relationship to a whole new level by exploring their mutual attraction in a way that's both passionate and in - line with their characters. There are also some that are more focused on the emotional build - up before any physical relationship, like 'The Slow Burn'. It gradually unfolds their feelings for each other in a very believable and engaging manner.
The exploration of their different cultural backgrounds is also a theme. Giles is from England with a very different upbringing compared to Buffy's American experience. Fanfictions might delve into how these differences play out in their relationship, whether it causes misunderstandings or brings them closer together. There are also fics that look at how they might interact in a more modern - day setting outside of the supernatural world, just as two people with a complex history trying to navigate normal life.
One common theme is the mentor - mentee relationship. Giles is Buffy's watcher, so a lot of fics explore how he guides her in her slayer duties. Another theme is the idea of family. Since Buffy doesn't have a typical father figure, Giles often fills that role in fanfiction. Also, the theme of sacrifice is there. They both are willing to sacrifice for each other and the greater good, and this is often explored in different ways in the fanfiction.
Well, I really liked 'Giles: The Early Years'. It gives a great backstory to Giles, showing how he got involved with the Council and his first encounters with the supernatural. 'A Day in the Life of Giles' is a charming short fanfic that just follows Giles through an ordinary day in Sunnydale, but with his unique perspective as a Watcher. There's also 'Giles and the Ancient Artifact' which has an exciting plot involving Giles and a mysterious magical item.
It's popular because it offers a different take on the relationship between Buffy and Giles. Their relationship has always been complex and adding a baby into the mix makes it even more so. It gives fans a chance to explore new dynamics and emotions.
One great Buffy Giles fanfiction is 'The Watcher's Daughter'. It explores a different dynamic between them, where Giles has a long - lost daughter who gets involved in the Buffy - verse. Another one could be 'Giles' Redemption', which delves into Giles' past mistakes and how he and Buffy deal with the consequences together. There's also 'Buffy and Giles: The Unspoken Bond' that focuses on the unspoken feelings and deep connection between the two characters.
You can find Buffy Giles fanfiction on fanfiction.net. It has a large collection of stories dedicated to various aspects of the Buffyverse, including those centered around Buffy and Giles. Another good place is Archive of Our Own (AO3). It has a well - organized tagging system that makes it easy to search for specific types of Buffy Giles fanfiction, whether it's about their relationship in a certain season or a particular AU (alternate universe).
One common theme could be the shock and adjustment. Given their relationship in the original series, the idea of them having a child is unexpected, so dealing with that shock is often explored. Another might be the protection of the unborn child, considering the dangerous world Buffy and Giles are a part of.
You can start by looking on Archive of Our Own (AO3). It has a large collection of fanfiction from various fandoms, including Buffy the Vampire Slayer, and you'll surely find a good number of Giles x Buffy stories there.