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.
I don't know the specific author of a particular 'Hells Angels Novel' without more context. There could be multiple novels about them written by different authors.