The idea of promises being made by Gabriel. Hopefully this re-occurrence, and what it means, will be picked up by the reader when Lucy makes a promise (The last one, and the only one not made by Gabriel)
"Half a lifetime" used literally in flashback ("It was, as he said, half a lifetime ago"), and metaphorically ("What felt like half a lifetime")
I was originally going to call it "Half a lifetime", but changed to "Promises" when I realised a promise was the McGuffin
(Though this is the first time I've thought of it as such).
Also four chapters from the end it is revealed why he is sometimes 'Gabriel' and sometimes 'Gabe'. Once you know why you can understand what he is thinking through the book.
Do the job in front of you.