Where Things Come Back is the title of the entire book and the title of the last chapter—you know, the one in which Gabriel reappears after his long disappearance. On the surface, it seems like maybe Gabriel's return is all the title's referring to, but if you look at little closer at the book, you'll see that people (and things) returning despite all odds isn't unique to Gabriel.
For example, the other exciting thing happening in Lily, Arkansas, during the summer of Gabriel's disappearance is the supposed reemergence of the extinct Lazarus woodpecker. Even though it turns out that the whole thing is a hoax engineered by a man desperate for attention, for a little bit of time, everyone in the town is reenergized by the fact that Lily might be a special place—the kind of place where an extinct bird would want to make its home.
We also see the storyline of Benton Sage, who goes on a missionary trip only to return home in disgrace, and that of Alma Ember, who goes away to college to escape Lily, but eventually winds up right back where she started. And on a broader level, the book ends with life returning to normal. It may not be exciting, but boring doesn't look so bad anymore, at least not to Cullen.