Symbolism, Imagery, Allegory
If the names of Luke and his brother sound familiar, that's because they are familiar to a huge percentage of the world: they're the names of three of the four Christian Gospels. Now, Mrs. Garner alludes to the four Christian Gospels as the origin of her sons' names, but she doesn't actually cite the Bible. Instead, she cites an old children's bedtime prayer: "Matthew, Mark, Luke, and John, bless the bed that I lie on" (2.38).
So if it doesn't have anything to do with the Bible, then why these names? There are plenty of popular quartets that could have been used for inspiration: Moe, Larry, Curly, and Shemp; Leonardo, Raphael, Donatello, and Michelangelo; John, Paul, George, and Ringo; Velma, Fred, Shaggy, and Daphne; take your pick.
Well, even if Luke's mom didn't know what she was doing, we're pretty sure that Haddix did. Basically, each Gospel account tells the story of Jesus from a slightly different angle, and the Greek word for "gospel" not only translates to "good news" but is also the root for the word "evangelize" which means "to preach or proclaim." The Gospels were meant to spread the word of Jesus and the hope of salvation—the freedom, if you will—that he was bringing to the world.
Luke may not have much to say about Jesus, but he sure has a lot to say about Jen. He swears to "tell everyone about" (30.39) her, promising to carry on her mission of helping free all shadow children and to keep her memory alive in the process. In other words, he's her gospeller.