Gabriel Witter always seems happy to be wherever he is, which is a trait that Cullen deeply admires in his younger brother. He loves the people around him without complication, and is always there to provide support and compassion to his friends and family. Cullen says:
My brother never seemed bored, and that only further angered me at the fact that I was most of the time unsettled and unfulfilled in everything I did. Gabriel was happy just reading a book or listening to music or walking around town with Libby Truett, his best friend. (1.23)
Even though people tend to like Gabriel, and Gabriel tends to like people, he is a solitary creature. Maybe that's because he doesn't feel the need to prove himself to anyone; he knows who he is and he knows what he enjoys:
Gabriel though, didn't really like going outside or swimming or anything like that. He liked to stay in, read books, watch TV, and pretend that he was grown up. (13.46)
Because Gabriel is so self-contained and well-loved, it seems horrible and unthinkable to his family and friends that someone would do harm to him. When he goes missing, it tears a hole in the fabric of their lives, and they desperately just want to bring him back home.
Anytime you come across someone named Gabriel in a book, it's probably a good idea to consider whether or not this just might be a shout-out to the angel Gabriel in the Bible.
In a book where religion is most definitely on the table (be sure to read up on Cabot elsewhere in this section), it's safe to say that Gabriel is definitely a biblical reference. Heck, Cabot even assumes he is the angel Gabriel after he kidnaps the poor kid—considering Gabriel's name, he thinks: "Gabriel, the Left Hand of God" (18.133)—plus Gabriel's bestie is Libby, which is a nickname for Elizabeth, who is one of the people the angel visits in the Gospel of Luke. So, yeah—Gabriel is definitely working some religious symbolism in this one.
The angel Gabriel shows up all over the place in religious texts, but for our purposes, Gabriel in our book and his religious counterpart represent the influence of unseen forces. Gabriel spends a sizable chunk of the book missing, after all, and yet even in absence he packs a powerful punch on the lives of several characters.