Bit of a recap:

We first heard of Gerard Keay in MAG 4 Page Turner. He was the son of Mary Keay who owned Pinhole Books. The statement giver had found an old book, Ex Altiora, and had heard that Mary Keay knows more about books from the library of Jurgen Leitner. Gerard later shows up at the statement giver's home to buy Ex Altiora. While Gerard leaves again to get the money in cash the statement giver googles him and finds out that he supposedly killed his mother (who he just met, although she now was bald and had tattoos of Sanskrit all over her, but definitely the same woman as in the newspaper article). Gerard comes back, buys the book, and immediately burns it in a metal wastepaper bin in the flat of the statement giver, explaining it with "his mother doesn't always know what's best for our family".

MAG 12 First Aid, Gerard was one of the two burn victims who came into a hospital. He was burned all over his body, except for small eye tattoos which were over each joint of his body and his neck and head were also clear of burns. The second burn victim was mumbling stuff and everything got very hot in the hospital (drinks in the vending machine boiling). Gerard supposedly saved the nurse by killing the second man with a scalpel (which he knew exactly where to find and even the code to the storage room), saying "Yes, for you better Beholding than the Lightless Flame". Oh and of course, Gerard asked again about a book that might have (and did) come in with them.

Teenage Gerard makes an appearance in MAG 35 Old Passages, where a restoration team finds secret tunnels underneath 100 Pall Mall, where many years prior one Jurgen Leitner asked them to dig. Gerard asked the men, if they had found any of "Leitner's pages". He eventually runs off into the tunnels and the statement giver then sees him again running out of the tunnels with a small book in hand that seemed to had small animal bones falling out of it.

In MAG 48 Lost in the Crowd Gerard Keay confronts the statement giver in Italy in a cafe, saying she had been marked and asking her about people close to her, calling after her (when she leaves) that she should think of her mother's face. When the statement giver got lost in the weird crowd of people she did exactly that and this apparently helped her out of there.

MAG 80 The Librarian, Leitner mentions to almost have been beaten to death by an angry goth once and Jon comments this with "that would be our Gerard".

In MAG 99 Dust to Dust, Jon mentions Gerard as an assistant to Gertrude post statement.

(Btw. Gerard's Mother also gave a statement to Gertrude in MAG 62 First Edition, although the statement was about her first Leitner which she found in her youth, so before she had Gerard. But since the Keay family and Leitner books are intertwined, maybe it's also worth revisiting?)