Stiles' father could be a potential rescuer. He's a cop and would be determined to find his son. He might use his police resources and connections to track down any leads on Gerard and Stiles. Also, the pack that Scott is a part of would all be likely to help. They see Stiles as part of their group, and they wouldn't let one of their own be in Gerard's clutches for long. Even some of the more minor characters in the show who have a connection to Stiles might step up, like the Sheriff's deputies who could unknowingly stumble upon a clue that leads to Stiles' rescue.
His friends, of course. Scott would be one of the most likely rescuers. As Stiles' best friend and a werewolf, he has the strength and loyalty to track Stiles down. Lydia might also be involved. With her banshee powers, she could sense Stiles' distress and lead the others to him. Another possible rescuer could be Derek. He has a lot of experience dealing with dangerous situations and might feel a sense of obligation to save Stiles, especially if he realizes Gerard is involved.
In this type of fanfiction, it's possible that a new character could be introduced as the rescuer. Maybe a powerful witch who has been watching Stiles from afar and decides to intervene. Or an old acquaintance of Gerard's who has a change of heart and helps Stiles escape. However, the core group of Stiles' friends and family still remain the most likely candidates. They know him best and have the most motivation to get him back safely. For example, Scott's loyalty to Stiles is unwavering, and he would go to great lengths to save him, using his werewolf abilities and his knowledge of Gerard's past actions.
Chapter 22, "Number 7203", is the clearest early answer because it turns Xiang Hao's situation into a concrete plot event. Early on, Hearing this, Su Yinwan looked at Shang Shaojing with a face full of.... By the end, Jiang Li's righteousness temporarily shut Jiang Yan up, but after a....
Chapter 426, "How Did Someone Fall Down?!", is the clearest early answer because it turns Xi Yang's situation into a concrete plot event.
Chapter 14, "She Looks So Cool Dancing with a Gun", is the clearest early answer because it turns Dingyuan Marquis's situation into a concrete plot event. The chapter turns that into something concrete by moving from Lin Manru had a rare good night's sleep.
Chapter 346, "Mighty Soldiers, One Against Three, The Cause of the Beast Tide", is the clearest early answer because it turns Beast Tide's situation into a concrete plot event. It opens with five million!" "Silver is just one of the main materials used in the... and closes on Among the Six Armies, the first batch to rush up are all the....
Chapter 162, "Jiang - Boyfriend Power Explodes - Li", is the clearest early answer because it turns Wen Jiaojiao's situation into a concrete plot event. The chapter gives readers a real answer by opening on He Yun then realized what was happening and subconsciously wanted to...
Chapter 329, "Refining the Jade Dew Stone, Cold Element Festival Eve (2)", is the clearest early answer because it turns Xia Hong's situation into a concrete plot event. The setup is Li Xuanling's talent is even stronger than her father, Li Tianhua., but the pressure keeps building through As soon as she breaks through to the Cold Resistance Level, her basic....
Chapter 307, "I Am Sister's Real Younger Brother!", is the clearest early answer because it turns Sister's situation into a concrete plot event. The chapter gives readers a real answer by opening on From the certificates and trophies on the wall, it was clear that...
Chapter 51, "The Great Lodging Competition", is the clearest early answer because it turns Chixia Village's situation into a concrete plot event. The chapter gives readers a real answer by opening on Oh, okay." After uttering two syllables indifferently, Jiang Li bent...
Chapter 170, "This Family Will Fall Apart Without Jiang Li", is the clearest early answer because it turns Su Yinwan's situation into a concrete plot event. The chapter turns that into something concrete by moving from Watching the two who had long remained silent, the PD shifted his...
Chapter 240, "He's the Last One?!", is the clearest early answer because it turns Su Yinwan's situation into a concrete plot event.