Spoiler warning for 8.05 I guess? Idk this sort of follows canon in that the Battle of King's Landing all happens as it did on the show, but from there it's all canon divergent. Also I know the timings are probably all wrong but I just made the best guesses I could with the few resources I have.

Title from Without Me by Halsey

Barely a week after Gendry arrives in Storm's End to a sea of confused and curious faces, his new maester comes to him with a raven from King's Landing. He's ashamed to ask the maester to read it for him, but there was little need for a blacksmith's apprentice to know more than basic words and it has never served him ill before. He supposes he'll have to learn now that he's a lord, but there will be time enough for that after.

The raven tells him that the war is won, Cersei and the Kingslayer dead, King's Landing razed to the ground. He feels a pang of something like sorrow at the news; he had always hated that stinking shithole of a city, but it had been his home for many years, and that has to mean something. Queen Daenerys does not yet sit the Iron Throne, but Gendry supposes that there is no more throne now that the Red Keep has well and truly fallen.

This information means little to him, however, as it does not give him the answers he desperately wants. Of course it doesn't - this is hastily penned, the kind of note that would have been sent to all the loyal lords and ladies of Westeros. Even so, he can't help but feel disappointed when there is no mention of Arya. He knows she would have been aiming for Cersei; he'd heard her repeat her list so many times that he feels it's ingrained in his memory as much as it is in hers. He hopes she is still alive, but he cannot keep himself from imagining her body laying broken and bloody in the ruins of King's Landing, or unrecognisable for burns made by dragonfire.

The thought makes him feel ill, so he brusquely thanks the maester and strides out of the room, barely conscious of where he's going. Somehow, he manages to find his way to his chambers, so he lies down on the bed and closes his eyes, trying to remember her smile and her beautiful grey eyes and the way she had looked hovering over him that blessed night they'd spent together. It works, for a time, but all too soon he's back to seeing her dead, and a heaviness settles deep in his heart. He eventually drifts off into an uneasy sleep, fraught with nightmares of dragonfire and death.

Two days later, he receives another raven, this one from Jon Snow. It differs from the other in that it is clearly intended for him alone, and, as the maester reads, Gendry begins to suspect that not even the Dragon Queen knew of its existence.

You have heard what happened to King's Landing, Snow writes. The Queen is not being well received and even those in her own ranks are beginning to have doubts. My father raised me to be honourable and just, and I cannot stand by and watch her burn cities to the ground. Some of our men died because of her dragon, and my sister, Arya, was in the city when it fell. We have found no trace of her, alive or otherwise -

At the mention of Arya, Gendry feels like the world has turned upside down. He staggers, just barely hearing the maester's voice falter and stop as he collapses into a nearby chair.

Arya. No trace.

She can't be gone. He doesn't want to believe it, but Snow's words ring through his head and he realises that his worst fears might be coming true after all. Arya, buried under mountains of rubble that won't be moved until she's cold and rotting. Arya, her skin red and hair gone so she's no longer recognisable to even her own family. Arya, dead.

He closes his eyes and leans his head in his hands, trying to calm his breathing. He hears a voice somewhere in the distance, or perhaps not so far away because it comes again, clearer this time, "My Lord?"

He jerks his head up, seeing the maester watching him in concern. "Are you unwell, my Lord?" he asks, and Gendry mentally berates himself for acting so foolishly. He nods and waves for the maester to continue reading, which he does after a moment's hesitation.

Our fathers were once great friends. I only ask that that alliance continues, in the hope that the worst can be avoided. Jon Snow, Warden of the North.

It takes a long time for the implications of Snow's words to fully sink in, and when they do, Gendry is shocked at what he is suggesting. It's on the tip of his tongue to say no; Daenerys made him a who he is now, and she can unmake him just as easily.

And yet. He's never really wanted a lordship, and his people barely know him; they wouldn't care if he left just as suddenly as he arrived. Besides, he had told Arya once that none of it meant anything without her, and her loss is hurting him in ways he never knew he could hurt. He wishes she were here, but that's not possible anymore. He'll never see her again.

The realisation hits him harder the second time around, so he sends the maester away and leaves for the forge. He likes working there, rules be damned; the sound of a hammer on steel is the only thing he knows now, the only thing that might keep his mind distracted. Sparks fly and metal sings, and Gendry images that he is killing every person who had a hand in her death.

He is woken one morning, not long after Snow's raven, by a knocking at his door. He climbs out of bed and opens it, raising his eyebrow at the young boy stood outside.

"There's someone at the gate, my Lord," he says quietly, his hands twisting together nervously. "Says she knows you, and that she has to speak with you."

"Who is it?" Gendry asks, frowning. He has received no message of a visitor, and can't think of anyone who might be coming unannounced.

"She said her name was...Arya Stark, my Lord," the boy replies, and Gendry think his heart might have stopped. He stares at the boy, searching for any sign of a lie, but he can see none, only fear and anxiety. Of course, Gendry's never been good at discerning lie from truth, but something in him is telling him to believe the boy, impossible though it sounds.

He pulls on his doublet and boots, then runs through the castle, nearly knocking the poor boy over as he passes him. His heart is beating fast in his chest and time blurs into nothing as he heads to the gate. The group of men gathered there part as he arrives, revealing a white horse, its rider stood next to it, and it is her. Her clothes are covered in a fine film of dust and her hair is matted and wild, but Gendry think he would know her anywhere.

He approaches slowly, staring at her in shock as it settles in that she is alive. More than that, she is here, with him at Storm's End, and he almost can't believe it. Then her eyes meet his, and, perhaps for the first time since they met, they are open and honest, and Gendry's heart breaks a little as he sees how exhausted she is.

"Arya," he breathes as he reaches her, his hand hovering just above her shoulder. He wants to hold her close to him, to kiss her and tell her how lost he's been without her, but he just barely manages to refrain himself. She smiles at him, a brittle and broken thing, then her knees buckle, and Gendry only just manages to catch her before she hits the ground. He signals to a couple of his men to carry her inside, then asks for the maester to be found and brought to him.

"Send a raven to Jon Snow, and one to Lady Sansa at Winterfell," he asks the maester. "Tell them that their sister is alive and safe at Storm's End. And tell Jon Snow..." He hesitates, then thinks of the girl sleeping only a few rooms away, the girl he has almost lost so many times over. Resolves settles hard in his mind and he turns to his maester with a grim look. "Tell him that I accept."

The maester's eyes widen at that, but he nods and heads off to do as Gendry asked. Gendry closes his eyes and breathes out slowly, clenching his fists at his sides to stop his hands from shaking. Opening his eyes, he turns and heads in the direction of Arya's rooms, silently swearing that no one will ever take her away from him again.

And gods help the person who tries.

This wasn't meant to be multi-chapter but whoops guess I'm going with it now. It shouldn't be too long, maybe three chapters, possibly a couple more.