Overall, the ending of Hells Paradise ties up most of the storylines. It brings closure to the conflicts and character developments, but leaves some room for interpretation and imagination.
You can start from the chapter where the anime ended. Usually, there are online forums or manga communities that can give you specific chapter recommendations.
There's a fanfic called 'Gabimaru and Sagiri: The Unlikely Alliance'. In this one, they start off as reluctant partners but gradually form a deep connection. The author does an amazing job of showing Gabimaru's protective side towards Sagiri and how Sagiri comes to respect Gabimaru's skills and determination. It has some really intense and emotional scenes that make it a great read.
Characterization is key. A good Gabimaru x Sagiri fanfic should accurately portray their personalities. Gabimaru is tough yet has a soft side, and Sagiri is strong - willed and determined. If the fanfic can show how they complement each other, it's great. For example, Gabimaru's combat skills and Sagiri's strategic mind working together.