Why is Jesus' agony in the Garden of Gethsemane missing from John's Gospel?
Matthew, Mark, and Luke record the Gethsemane agony because the synoptic Gospels emphasize Jesus' humanity and suffering. John's Gospel emphasizes His deity, so it omits that scene, just as it omits the temptations. This distinction explains why many synoptic events do not appear in John.
Les explains that Matthew, Mark, and Luke are called the synoptic Gospels and deal primarily with Jesus' humanity. John's Gospel, by contrast, deals with His deity. Therefore, many things covered in the synoptic Gospels are not mentioned in John. Les gives the temptations as an example: they are recorded in Matthew, Mark, and Luke but not in John. Likewise, the agony in the Garden of Gethsemane—and he emphasizes the agony of it—is recorded in all three synoptic Gospels but is not recorded in John's Gospel because that was part of His humanity, of His suffering.