Game of Thrones Season 7 is all about big meetings and big returns. One of those reportedly returning is Robert Baratheon's bastard son Gendry Waters, freaking finally, after he rowed away from Ser Davos at the end of Season 3. What happened to Gendry in the Game of Thrones books? While his story hasn't been quite as epic, George R. R. Martin does occasionally check in on him.
Gendry's origin in A Song Of Ice And Fire is the same. Ned Stark discovers him in King's Landing during his film noir detective storyline in Season 1. Then the blacksmith joins Arya and Hot Pie on the road, getting captured first by the Mountain's men and taken to Harrenhal, then by the Brotherhood without Banners. However, his current whereabouts are a little different — and a little bit more known. Instead of being kidnapped by Melisandre and Stannis Baratheon, he actually gets to join the Brotherhood and is knighted as Ser Gendry.
In A Feast For Crows, the fourth book, Brienne encounters Gendry working at the inn at the crossroads (where Arya just caught up with Hot Pie in Season 7) as a smith. Brienne is struck by how much he looks like her dude Renly Baratheon, and the two of them get involved in a fight where he kills a minor villain called Biter — who, again, Arya kills on the HBO series.
Also in the books, Robert has other living bastards, most notably a young woman named Mya Stone who Sansa encounters at the Eyrie. There is also a boy named Edric and a girl named Bella — who unknowingly flirts with Gendry and, for once in Westeros, he rejects what would be accidental incest.
I think it's important to note that, while it isn't often alluded to on the HBO series, Robert Baratheon has Targaryen blood. His grandmother was Rhaelle Targaryen — the daughter of Maester Aemon's brother King Aegon Targaryen V, who was of course the great-grandfather of Daenerys Targaryen. So that means that not only are Dany and Jon Snow related to each other, but Gendry is related to them too.
Even though Gendry has not been seen since the fourth book, much like the HBO series, George R. R. Martin's novels have a different pace. It's not cause for concern. That does, however, mean that Gendry's role in the upcoming war is unwritten. That makes whatever happens to him next on the show that much more exciting.