There's a point where Greg thinks, he's not going to be able to do this. Not at all. Look on your works ye might and despair.
Such things that Sherlock Holmes has made. And John, John who is here now and trying to deal with this absence.
He's not sure either of them are but Greg has Madeleine Nadiyah and a structure and some kind of a job he knows he can go back to once the enquiry and the fuss from the enquiry have died down. He has a routine and a child to look after and a house to manage. John, John for all his strength is slowly fading away again and so, though he's unsure if he can do any good he invites John to come and stay because at least it's something useful to give a bed and cups of tea and maybe a beer and an ear, if he should so need it.
He does not expect John to take to Maddie. He does, though. Greg finds himself teaching John how Maddie likes her food (mashed up peas, fruit pieces and a cup of milk now she's on solids), bed time (bath, story, lullaby, night light, sleep (if he's lucky)) and the way Maddie stays still if you clap your hands with her. And they get to know each other over getting thrown up on and ridiculous late nights. And John even seems like he might be somewhat better, if anything can be better in this whole mess (Greg doubts this is the case, but he's holding on to what he can get for both of them).
In the first weeks after John makes tea because it's what you do. Except he makes two cups of tea - one without milk and sugar and one with both. He ends up dumping them both unwashed in the sink to join the other piles of cups and going to work in the hope that it might help. As it turns out he can see Sherlock in ever skeleton, every patient with a case that is even somewhat odd. Every time he walks past the morgue in the hospital he finds work at. He finds himself walking and walking and walking until he can pass out from tiredness or he'll take every possible shift because at least at work he has something to do while his mind is with Sherlock in a thousand and one different ways.
When Greg offers him a room he nearly says no. Nearly says, what's the point, because we've lost the man we had in common, why do you care? But he goes because in the end it helps him feel useful, to have someone who wants to help him. Also he doesn't think he can face any more of 221B (Mycroft or someone keeps paying the rent and everything is still in place, which is a whole other level of.txtredibly fucked up, John knows that, but it's better than having to clean it out) or a bedsit by himself or Harry's place, which would be entirely unhealthy for either of them.
So he goes to Greg's and finds himself a person to care for. It's Greg's daughter. He'd never paid much attention to her before because he'd never really known much about Greg beyond what Greg volunteered. He learns now, though. He learns that Greg sings to his daughter in French, that he cooks the best crepes he's ever tasted and sings all kinds of things re pitched as lullabies. He knows Maddie's mother and Greg stay in touch as much as possible (she'd gone to the USA for work, long term) and that they are planning a visit soon but that they both agreed that Maddie was going to stay in England, where grandparents and extended family are just a channel or a train ride away. He knows that Greg misses his ex wife, though he doesn't speak of it.
Just as he doesn't speak of missing Sherlock. Neither of them do, beyond that first long night when Maddie was at her Nan's and they both nursed beers and gave Sherlock Holmes something of an eulogy. Instead they talk of common place things and banter and try not to think of the missing third man who would find all of this.txtredibly dull. Or maybe he wouldn't John thinks. Maybe he'd think Maddie the most fascinating being in the world. They'll never get a chance to know now.
It turns out that Sherlock is fascinated by Maddie.
But that's later.
After they've both gotten over the shock and the anger (and in John's case the urge to punch the man in the face several times and scream because he can't believe this has happened and this is what Sherlock has done, damn him. And Mycroft. Especially Mycroft, who kept this secret for so long and in the face of so much grief, though he supposes he shouldn't have expected anything different). After they've found Moran, holed up on a rooftop opposite 221B, sniper rifle in hand as he fires at a model of Sherlock.
John wonders if Moran was made sloppy by grief, though he can't imagine why.
Later. Later they'll unpack what it all means - King Arthur, Sir John and Sir Sherlock (Greg calls Sherlock 'Merlin' then and John decides he really needs to brush up on his Arthurian legends, if only to puzzle out who the hell he's supposed to be). And they form a guard around Maddie, who has no idea of any of this.
Sherlock talks to her like an adult, but it's always with love, which surprises and doesn't surprise John. Sherlock, who lets Maddie gum on his scarves and coat collars, heedless of the cost. Sherlock, who'll play the violin for her any time. Sherlock who'll look at her in fascinated love for hours and hours. Sherlock who lets Maddie nap on his chest.
John has always known he has loved Sherlock. That he loves Greg takes longer to come to him. That they both love him and love each other? That comes much later. Sherlock is hesitant, scared (of what he thinks he cannot be, of what the world has told him he should not be) and it never occurs to Greg until John points it out to him and Sherlock tells him.