Game Of Thrones Season 7: 7 Theories On How Gendry Will Return
7. He Made It To King's Landing
Assuming it was all (literally) smooth sailing for Gendry, then he will have ended up exactly where Davos intended: King's Landing. It is technically his home, as much as he's ever had one anyway, and the place he knows better than anywhere else.
His aim was to make it to Flea's Bottom and relative safety away from Melisandre and her leeches, so what if that is just what he accomplished? No great mystery here, just a bastard keeping his head down in the big crowds of people and slowly getting by. It's not exciting in terms of plot, but it would fit with the position Gendry was in when we last saw him.
Gendry didn't display much desire to be the hero, only to have something resembling a family, so maybe he reached Flea Bottom, made a few friends (or just kept himself to himself, for safety purposes), got some work as a smith, and has lived with his lot.
However, he's got careless, thinking he was totally safe now, and is discovered - Cersei would no doubt want to immediately take out a Baratheon bastard - or runs into an old friend that forces him out of hiding.