Gendry’s second name is not “Waters”.
He is not an acknowledged bastard, and thus doesn’t get the second name, unlike Jon Snow and Edric Storm. Please stop calling Gendry, “Gendry Waters”.
Gendry is known by several people to have a high-born parent, thus he is entitled to the name Gendry Waters. He doesn’t use it because he doesn’t know his parentage, but that doesn’t mean the name isn’t his. He is Gendry Waters, though I doubt he’ll ever use that name.