He originated from the Zala branch of the extended gens (clan) Péc, which had large-scale possessions in several counties of Transdanubia, in addition to other parts of the Kingdom of Hungary.
[4] Gregory is first mentioned by a royal charter in 1266, which refers to him as ispán of Rojcsa (today Rovišće, Croatia), which laid in the territory of Križevci (Kőrös) County and belonged to the dominion of Béla, Duke of Slavonia.
[6] Historian Attila Zsoldos considers the intention behind the appointment of the young Gregory as leader of one of the involving armies was to give him opportunity to demonstrate his military skills.
Duke Stephen, a talented military leader, has been trying to build personal relationships with those noblemen, who arrived to Bulgaria from his father Béla's dominion.
[7] His political career reached its peak during the reign of Ladislaus IV of Hungary, whose rule was characterized by feudal anarchy, when many groupings of barons — primarily the Csáks, Kőszegis, and Gutkeleds — fought against each other for supreme power.
Gregory Péc became Ban of Barancs and Kucsó (present-day Braničevo and Kučevo in east-central Serbia, respectively), first mentioned in this capacity on 27 November.
His inherited and acquired lands laid in the borderland of domains of two oligarchic powers, the Csáks and the Kőszegi family, thus his political, social and financial status were constantly at risk.
The Peace of Hainburg, which concluded the war, was signed on 26 August 1291, and three days later Andrew and Albert of Austria confirmed it at their meeting in Köpcsény (today Kopčany, Slovakia).
When Andrew III and Albert of Austria jointly invaded the Kőszegis' lands and their main fort at Kőszeg in August 1296, Gregory participated in the military campaign.