Gregory was first seen and mentioned during the State Address at the Palace of Westminster, where he was hailed and talked to, apparently to Gregory's disdain by Arthur Underwood. Based on the fact that he attended the Address, it is likely he was a minister of some sort.
His was last mentioned when Nathaniel used his Scrying Glass to spy on his master, Underwood, who happened to be speaking with him on the phone. They were discussing business at the time, though Underwood also asked him a personal question about what he should do with Nathaniel after he had found he attempted to summon a Djinni (This being Bartimaeus).
It is not known whether or not he attended the government entourage at Heddleham Hall, or if he did whether he survived the wrath of Ramuthra. He is not mentioned in any of the books after The Amulet of Samarkand