
born sometime in the 1980s; I'm going with February 1986.
1988-89, date unknown (age 2-3) - Gerard's father is murdered by his mother and bound into the book. Gerard mentions that his father decided to leave after he was born, and Eric himself says that he thought about how to do it for two years, and that he wasn't killed until several months after he left. So Gerard was probably around 2 and a half at the time.
2002, March (age 16) - Events of MAG 35; retrieves a Leitner from an underground tunnel
2008, July (age 22) - Mary Keay gives his father's page to Gertrude, who summons him and agrees to burn his page, and also to make sure his son is okay
2008, late September (age 22) - Mary Keay binds herself to the book; Gerard is taken into custody on suspicion of his mother's murder and is eventually acquitted
2009, September (age 23) - Grudgingly helps Andrea Nunis in Genoa, Italy; Jon mentions this was likely shortly after his acquittal
2009-2011, date unknown (age 23-25) - Gets his eye tattoos. Andrea Nunis saw him close-up in 2009 and does not mention them despite giving a detailed physical description, but he has them by December 2011 when he is admitted to the hospital.
2011, December 23 (age 25) - Gerard is admitted to the hospital with burns covering 80% of his body, seemingly while in pursuit of a Leitner of some kind. While there, he kills Diego Molina, and is released into his mother's care several days later.
2012, November (age 26) - Gerard buys Ex Altiora for 5000 pounds and burns it.
2013, date unknown (age 26-27) - Gerard meets Gertrude. The timeline around that gets a little fuzzy, but here's what we know:
1) Gerard says his mom haunted him for five years before Gertrude freed him, and she "died" in Sep. 2008.
2) Gerard was already in contact with Gertrude when he found Leitner (see below) in what was probably late 2013 or early 2014.
3) Gerard says he joined Gertrude's work "for a few years."
We know Gerard died in late 2014, and the very earliest he could have possibly met Gertrude was November 2012--but that would have been much closer to four years since his mother's "death" in Sep. 2008, so my guess is that they actually met in early 2013. They probably worked together for something like 18-22 months, which is hardly "a few years," so Gerard was probably either exaggerating or overestimating the amount of time. My personal interpretation would be that, since she's probably the only person he was ever any amount of close with, it felt longer than it was.
2013-14, date unknown (age 27-28) - finds Leitner, who says in Feb. 2017 that it happened "about 3 years ago," so assuming late 2013/early 2014; Leitner describes it as being beaten nearly to death by an angry goth, whereas Gerard describes him as a pathetic old man and says he just "let him go," so the truth is probably somewhere in the middle
2014, early June (age 28) - in France alongside Gertrude
2014, late June (age 28) - Gertrude is in China; Gerry is presumably with her
2014, early July (age 28) - Gertrude is in New Zealand, Gerry shows up and they both leave in a hurry
2014, July onwards (age 28) - seems to have been in the U.S. with Gertrude until dying in Pittsburgh sometime in late 2014; it must have been after October 9th, because Gertrude speaks of him in the present tense during a statement from that date.
Gertrude binds him into the book while his body is in the morgue, during which she is arrested. Though she talks her way out of the charges, the book is left in police custody.
2015 (undead) - the book disappears from police custody sometime in the latter half of this year, probably stolen by Trevor and Julia
2017, June 30 (undead) - Gerard talks to Jon
2017, early August (undead) - Jon burns Gerard's page, granting him actual death