Backstory
[Katniss says goodbye to Gale after his involvement in the bombing that killed Prim. ]
Gale: "No Katniss, please don't leave me! not after everything we've been through! Please understand!"
Katniss: 'I'm not your best friend anymore...I can't do this... goodbye Gale."
Gale cries and falls to his knees. Katniss coldly just leaves the room and shuts the door.
Epilogue
Katniss and Peeta move forward with their lives, finding happiness together. Gale, however, goes to District 2, unable to forget Katniss and struggling to let go.
One year later*
Gale visits Katniss's home and knocks on the door. Katniss opens the door Gale says with a soft voice, "It's been a year, Katniss. I really, really miss you." " Katniss responds, "Gale, it's over and I've moved on." and closes the door
Two years later*
Gale returns, still consumed with thoughts of Katniss. He knocks again. "It's been two years, Katniss. I really, really miss you." Katniss sighs and shuts the door in his face.
Ten years later*
Gale's obsession has taken a darker turn. He carved his eyelids out so he would never sleep again, and etched a blood-red smile into his face with a sharp knife, attempting to convince himself he's happy, just as he once was when he and Katniss were best friends. His appearance has drastically changed and his voice became deep and raspy. He had been driven to the brink of insanity due to his separation and loss of Katniss. Instead of knocking on the door, Gale taps on her window with his knife. Gale says with a low voice, "It's been ten years, Katniss. I really, really fucking hate you. I know you left me, but don't worry... I know where you are. I'm going to hunt you down and kill Peeta. You better watch your back. Hehehe... HAHAHAHA!" Katniss and Peeta look at him in shock and decide to flee
Twenty years later*
Gale arrives and visits Katniss' house, but her house is empty, and she and Peeta are no longer there. It had been believed they moved somewhere else after years of being stalked. Gale then goes on a journey to find Katniss and Peeta
A century later*
Gale's physical appearance and height remains unchanged. Gale has spent the last 80 years wandering aimlessly, still searching for them. He hadn't shaved or showered in years and looked very unkempt. Katniss and Peeta are believed to be dead by then, at least that's what normal people would think. But Gale isn't normal anymore, he doesn't have even a tiny bit of humanity within him anymore. Gale says aloud to the air, "It's-it-it-it-it's been 100 years, Katniss. HEHEHEHEHEHAHA! I know you're still alive out there somewhere. Hehehe. You've been hiding for long enough. You can run run but you can never completely hide from me. My old self has been dead for the past eighty years. I think it's time for you to come back. Hehehe. I think I'm immortal because I'm still the same as I was eighty years ago. Come on back Katniss! Hehehe." With his final words and maniacal laughter, Gale takes his knife and plunges into himself to end his suffering and insanity