When Gavin tells Jack about his first encounter with Elliot, Jack doesn't believe him at first and then suggests to call the forestry in the morning. Gavin fails to mention the three other eyewitnesses as well as the pickup, which would have certainly been damaged when Elliot stopped it, thus proving his story. However, as we learn later, his goal was to capture the dragon and take credit for it, so he needed help from someone who he could trust. Realizing that Jack isn't interested in his goal (and feeling hurt about it), Gavin walked away in anger, not even wanting to convince Jack of anything anymore.