One possible story is that Gabe was drunk and got into a rage, attacking Percy. Percy was unable to fully defend himself, and thus got scars from the encounter.
Well, it could be that in the fanfiction, Percy gets into fights with Gabe over protecting his mother. Gabe, being the brute he is, might use a weapon or his fists, and Percy ends up with scars as a result. For example, he could try to stop Gabe from hitting his mother, and Gabe turns on him, causing injury that leaves scars.
In the fanfiction, Artemis might use her divine powers. Maybe she creates a distraction so that Percy can escape from Gabe. For example, she could make a sudden bright light or a loud noise that confuses Gabe, giving Percy the chance to run away.
Maybe Artemis sees Percy's potential. Percy has shown great courage and skill in other situations, and Artemis might recognize that he could be an important asset in the future battles against the forces of evil. So, she decides to save him from Gabe, who is a nuisance and a threat to Percy.
They might react with shock, as they may not have fully realized the extent of the danger Percy has faced. Some gods could be impressed by his survival and show newfound respect.
In many fanfictions, the hunters would first assess the situation. If Percy is being held captive by Gabe, they may send in a stealthy member to find Percy's exact location. Once they know where he is, they would launch a coordinated attack. The hunters are known for their teamwork. They could surround Gabe's place, and some might engage Gabe directly while others free Percy. Their connection to nature might also give them an edge, like using the environment to their advantage during the rescue.
Jason might be shocked at first. He could feel a sense of respect for Percy, knowing that those scars are the marks of his many battles and hardships. Maybe he would be more curious about Percy's past adventures that led to those scars.
The scars might be a symbol of Percy's loyalty. He has fought for his friends and the world, and the scars are the physical reminders of his selfless acts. In the fanfiction, when Jason sees them, he might understand Percy's true character better. Also, the scars could be a connection to Percy's past loves and losses. They might have stories related to people he cared about deeply and lost, or relationships that were strengthened through the battles that left the scars.
One possible scenario is that the hunters disguise themselves as normal people and approach Gabe's place. When Gabe least expects it, they reveal their true identities and take him by surprise, saving Percy. Another could be that they use their tracking skills to find Percy's location even if Gabe has hidden him well.