Gregory is a strict mentor, always to the point and hard to please. However, he gives praise to Tom at some rare moments, albeit rather gruffly and (somewhat) reluctantly, showing that he has a soft side (but is afraid to show it). He claims this lack of praise is at least in part to do with him doing his best to avoid Tom becoming bigheaded and making mistakes.
He has had relationships with Meg Skelton and Emily Burns, and is suspected to have more. He met Emily Burns first, admitting that she was his brother's girl, claiming that "he was young". However, she got tired of him and moved on, although they remained good friends. He says she had already got over his brother, one of the reasons why their relationship started. When he had just become a Spook, Gregory met Meg. He had been going off to take care of a monster, and she happened to be in the tower with it, bound with a silver chain. He ended up sleeping with her and discovered that she was a Lamia witch in the morning (after noticing the line of yellow and green scales along her back). When he tried to put her into a pit, Gregory found he couldn't bring himself to do it. Eventually, he sends Meg and her sister, Marcia, back to their home in Greece at the end of the third book (The Spook's Secret).
He has a brother named Andrew, a skilled locksmith. He has made keys that can open almost any door for both the Spook and Tom, which have often come in use, such as when battling the Bane in the Priestown catacombs. Tom meets him for the first time in the second book. He has one other significant brother who is a priest. He is first met in Horshaw after Tom has just begun his apprenticeship. Gregory and his brother did not speak, because Emily Burns was once the priest's lover, but went for the Spook instead. However, he did decide to pay his last respects and attend his funeral when he died. He died afer he was attacked by a ripper (a powerful boggart).During the fifth installment, he sends his apprentice to train with Bill Arkwright to receive further training against the everlasting battle against the dark. Later he has to help Tom rescue Arkwright from several Water Witches. At the end of the book, he sends Alice away because, among other things, she used a mirror to communicate with other witches, including the witch assassin Grimalkin, who ended up saving Tom's life in the fifth book.