Ultimately, outside of organized play scenarios like Pathfinder Society or (for D&D) Adventurer’s League, the GM isn’t really obligated to do anything; they are welcome to run the game as they like, and as a player your only “real” right, that is, the only one you can protect and enforce yourself, is the right to not play.
Is the treasure written into the adventure put there with the expectation that the player characters will receive it? Yes. Is it going to change the way the adventure plays out if the GM changes it? Also yes. Is there something wrong with that? Well, there’s something wrong with that if there’s something wrong with it.
To a certain extent, when a GM says “I’m running Giantslayer,” you as a player reasonably have certain expectations about what that game is going to entail, based on the stated premise of that adventure. If the GM deviates too substantially from the written adventure, then you could wind up with something quite different from what you expected—and if the differences are negative in your opinion, you might reasonably say you think there’s something wrong with how the game is going.
I doubt you’ll get a whole lot of sympathy from anyone if you say “well I read the adventure myself and I know that encounter was supposed to drop this item and it didn’t and that ruins the experience for me.” Most people would consider that a minor change and not one to be terribly concerned about (as, indeed, you didn’t seem to be, since you didn’t say anything and kept playing).
On the other hand, “the GM is so incredibly stingy with items that I feel like we’re struggling to even just barely survive, and I know this adventure and it’s not supposed to be like that, it’s supposed to be a more upbeat, epic adventure fantasy,” is probably something more people are going to appreciate. If the changes the GM is making to the written adventure are turning it from a fun, action-packed adventure of heroism to a grim, gritty, slog against an uncaring world, I think a lot of people would understand not being happy about it, and feeling like the GM had pulled a bait-and-switch on you. (The reverse would also be true: a GM giving out tons of loot in what’s supposed to be a grim and challenging adventure and turning it into a cakewalk could result in much the same kind of disappointment. It’s not about one versus the other, it’s about what you were reasonably expecting based on what you were told upfront not matching what you actually are experiencing in play.)
But ultimately, other people’s sympathy isn’t really relevant either—the only people who really matter are the people at the table. It only matters if the GM sympathizes, and understands your concerns and is willing to adjust their style to match yours. If they don’t care, or if they do but, apologetically, aren’t going to change from their own preferences, you’re back to your only option choice being “should I stay or should I go?” The feelings of the other players matter a lot too—if you’re the only one bothered by it, the table probably isn’t going to reorient the campaign to match your preferences. If they are all bothered by it, that might induce the GM to change. Or it might cause you all to collectively realize that what you each want from the game are too different to sit down and enjoy the same game—which is a shame, but often “no gaming is better than bad gaming.”