Game Of Thrones Season 7: 7 Theories On How Gendry Will Return
5. He's Going To Claim The Throne

Let's say Gendry made it to King's Landing, and has been living there ever since. He would've had a close-up view of the destruction Cersei Lannister caused, with the Sept of Baelor engulfed in green flame, and her taking the Iron Throne for herself afterwards. Even if he wasn't in King's Landing at the time, but merely somewhere fairly close-by, word would spread like, er, wildfire, and he'd hear of the Mad Queen sitting on the Iron Throne.
Gendry may not want to be a hero, but he's also not someone to stand by while so many innocent people get hurt. He knows how dangerous Cersei is, and could well want to put a stop to that.
He'd have a better chance than most, and more incentive too, because he himself possesses a fair claim to the Throne. As a bastard of Robert Baratheon, he may decide that the only noble thing to do in order to protect the realm from what is - as far as he knows - the biggest threat is to make a bid to be King, bringing him right back into the game as a major player.