I mean, that scene is word-for-word from the book, so don’t blame the movie! :) Yes, Gus is super pretentious at the start of the story. it’s a character flaw.
Gus wants to have a big and important and remembered life, and so he acts like he imagines people who have such lives act. So he’s, like, says-soliloquy-when-he-means-monologue pretentious, which is the most pretentious variety of pretension in all the world.
And then his performative, over-the-top, hyper-self-aware pretentiousness must fall away for him to really connect to Hazel, just as her fear of being a grenade must fall away. That’s what the novel is about. That is its plot.
Gus must make the opposite of the traditional heroic journey—he must start out strong and end up weak in order to reimagine what constitutes a rich and well-lived life.
Basically, a 20-second clip from the first five minutes of a movie is not the movie.
(Standard acknowledgement here that I might be wrong, that I am inevitably defensive of TFIOS, that it has many flaws, that there’s nothing wrong with critical discussion, and that a strong case could be made that I should not insert myself into these conversations at all.)
I really like this explanation of Gus. Mostly because it’s so painfully true.
In the beginning, Gus is the image of confidence in a way that makes him seem a little less than human. Which, interestingly enough, was his entire goal. Within the book, it breaks for him, chip by chip to turn him more into the damaged boy he is rather than the forever-etched-in-the-world man he wants to be.
In a way, both main characters have to weaken their independence to the point they can rely on another rather than acting ‘tough.’