Actually, two things.
1. Gendry does make another appearance in A Feast for Crows, when he's working at the Inn at the Crossroads and is affiliated with the Brotherhood. Brienne runs into him, I think.
2. Edric Storm is one of Robert's noble bastards who disappears. On the show, he was merged with Gendry, since they basically occupy a similar position. He escapes from Melisadre after Davos lets him go. Then, if memory serves, he completely disappears from the story. He's the one with the backstory about Robert banging some noble lady at a banquet or whatever, but it's only really relevant because he's one of Robert's bastards (along with Mya Stone in the Vale). I think the reaction to "ooh, Edric Storm has noble blood!" is more only relevant to the characters in the story because of the scandal associated with his birth. Otherwise, I don't see his lineage ever having come into play.
That said, I do think that GRRM revels in worldbuilding. The sheer volume of backstory and history for the Seven Kingdoms and most of Essos is staggering. While some of it has undoubtedly been expanded upon by other authors in the World of Ice and Fire book, Martin himself has included a fair bit of stuff. In a way, I appreciate this because it makes the world itself that he's created feel more real. The story exists beyond these characters in the books that we follow. Moreover, the characters all have motivations and attitudes and beliefs which are directly tied to the world and its history. All of that is to the good.
But as has been said here before, Martin does tend to get lost in his garden from time to time and forget about the azaleas that he showed us a while back, because he's grown enamored of chrysanthemums instead.