I confess I was entirely ignorant about TGG, never even having read F. Scott Fitzgerald's novel. I was struck by how much it echoes Alain-Fournier's Le Grand Meaulnes, a novel I know because I was in a beautiful stage adaptation of it back in college. (I played the title character, Augustin Meaulnes.)
The similarities only begin with the titles. In both books, a narrator relates the story about his dynamic, dashing, daring friend. Said friend met a young woman in the past, it was a life-changing moment, and he is now obsessed with "recapturing the past" and trying to bring himself and the woman together. The narrator stays mostly passive throughout the plot, although he does what he can to encourage and help his friend.
From Googling around, I note that I am far from the first person to notice the similarities between the novels. Fitzgerald spent a lot of time in France; he must have read LGM at some point during the 12 years that passed between the publications of LGM and TGG.