Brendan and Brandon Starr walked through the halls, looking for someone else to change with the watch, and the next person they saw was Gavin, the nerdy gay boy who (along with four others) had seen the video recording of Mark turning into Walter and Simon's transformation into Brandon. So, when he saw the pocket watch that Brendan was holding, he recognized it immediately.
"Hey, faggot," Brandon said, walking towards Gavin. It pained the nerd to hear the jock calling him that. He and Simon were close friends and he never teased Gavin about his homosexuality.
When they got close enough, Gavin made a split decision. Obviously, they were intending on using the watch on him. He couldn't run away, so his only option was to try and steal it from them. If he got his hands on it, then he could change them and he'd be safe.