I don't think that was Gwynn. It seems unlikely his voice would carry like that, especially to the mic on a cellphone or camera, with him facing the opposite way. I'm pretty sure it was someone in the stands, mimicking his "nervous twitch" glove movements.
When the video begins, you catch just a short burst of that person's voice saying, "Hey, don't act like you're playing" right next to, or maybe even holding the camera. Plus, the voices/laughter right around the camera, is the same decibel as the voice that's supposedly Gwynn. You also hear that same voice again, as Gwynn runs away from the camera, and the decibel never changes.
Therefore, there was obviously NOT a second heckler, behind the picket fence at the top of the grassy knoll.