Create a believable scenario for the pregnancy. It could be something that happened unexpectedly but that they both come to accept. Then, focus on the emotions. Show how they feel about the situation, whether it's fear, excitement, or a mix of both. Also, add some conflict. Maybe there are those who oppose the pregnancy or there are difficulties during the pregnancy itself that they have to overcome.
One idea could be exploring how Guts and Casca adjust to the news of her pregnancy. Maybe Guts tries to become more protective and less reckless in his battles.
In some fanfictions, their relationship can be seen as a source of inspiration. Griffith's grand vision might inspire Guts to strive for something greater than just survival. At the same time, Guts' unwavering determination can be an inspiration for Griffith to keep pushing forward in his own goals, all without any romantic or sexual elements.
Guts is purely fictional. It's a concept or character that exists within the realm of imagination and creative storytelling rather than being based on reality.