Gergel made his the Corgoň Liga debut in the 7–1 defeat against MŠK Žilina on 14 July 2007. After his first season he relegated with Trenčín to the Second Division. He scored 10 goals in the 2008–09 season and 3 goals in the 2009–10 season. In September 2010, he signed four-year contract for MŠK Žilina for an undisclosed fee. He played four games for Žilina at the 2010–11 UEFA Champions League group stage.
Gergel was first called up to the senior national team for two unofficial friendly fixtures held in Abu Dhabi, UAE, in January 2017, against Uganda and Sweden. He capped his debut against Uganda, being fielded from the start until the 70th minute, when he was substituted for Miroslav Káčer. Slovakia went on to lose the game 1–3. Gergel also appeared in the second half of the match against Sweden. In the 60th minute he substituted Káčer. Slovakia lost the match 0–6.