The theme of redemption can also be common. Maybe the gunslinger has a troubled past and is trying to make up for it in the RWBY world. Friendship is also a big theme. As the gunslinger interacts with the RWBY characters, they form bonds that help them grow and face challenges. For example, they might start off as a loner but learn to trust and rely on others.
One great 'rwby gunslinger fanfiction' is 'The Gunslinger of Remnant'. It has an interesting plot where the main character, a gunslinger, gets involved in various adventures in the RWBY universe. The story is well - written and the action scenes are exciting.
First, give them a clear backstory. Maybe they were trained by an old gunslinger who had a connection to the supernatural world. This backstory can explain their skills and motivation. For example, if their mentor was killed by a devil, they would have a reason to be involved in the Highschool DxD world. Second, define their personality. Are they a lone wolf or do they like to work in a team? Their personality should be consistent throughout the fanfiction. Third, give them unique abilities related to their guns. It could be something like the ability to shoot bullets that can track their targets or bullets that can create barriers.
The information provided by the gunslinger TV series did not mention that the series had 40 episodes, nor did it mention that it could be watched for free. Therefore, I don't know if the gunslinger TV series has 40 episodes and if it's free to watch.