He only shows up if the last leader of the Goblins is killed, while one or more party members are at the Grove at the moment, when the last of the three Goblin leaders dies.[1]
The death of the last Goblin leader causes the state of the Grove to instantly change to one where the Tiefling refugees are packed up and about to leave for Baldur's Gate. Gerson exists to ensure that this potentially immersion-breaking transition is not visible to the player. Engaging any party member back at the Grove in conversation prevents the player from moving the camera around in that area while the transition occurs.