Peter Parker otherwise known as Spider-Man, received an unsent letter written years ago by his late girlfriend, Gwen Stacy. He could tell that there were faint traces of writing on the other side, so he had tests performed on it by a forensics lab.
The tests revealed text stating that Gwen had been pregnant and had twins. He collected DNA from the grave of Gwen Stacy and prooved that the twins, Sarah and Gabriel (who had been threatning and attacking him and his family) were her children.
Mary Jane then revealed that she had known for years that the father of Gwen's children was Norman Osborn, the Green Goblin. He got so enraged that he broke everything within reach. He met Sarah and Gabriel on the Brooklyn Bridge, the place of Gwen's death, to tell them that Osborn had been manipulating them. Angered, Gabriel remained convinced of Osborn's lies that Peter was their father, and that he had abandoned them. Sarah, on the other hand, believed Peter. Gabriel then attacked Spider-Man, but Peter refused to fight back since he didn't want to hurt Gabe. Police then shot at them, wounding Sarah and causing her to fall off the bridge. While Spider-Man rescued her, Gabe went underground to an area predetermined by Norman Osborn. There, he discovered that Peter had been telling the truth. He injected himself with the Goblin Formula and donned a gray version of the Green Goblin costume, becoming the Gray Goblin. Sarah shot at him (to protect a weakened Spider-Man), causing the Glider to blow up. He landed on a beach, with amnesia. He later returned home to Paris where Sarah secretly cared for him. While his sister left to be trained as an Interpol agent, Gabriel was last seen in a crazed state fleeing the Osborn estate.