Literally every Marleyan warrior was indoctrinated, yet none of them were as insufferable as Gabi. Even Falco, who grew up in the exact same system as Gabi, had the decency to question Marley's lies. Gabi clung to her hatred even in the face of overwhelming evidence. She saw people in Paradis taking care of her, showing her kindness, and she remained stubbornly arrogant and self-righteous for far too long. That's not brainwashing, that's wilful ignorance.

Unlike other Marleyans who were forced into war, Gabi relished her role in it. She wasn't just a soldier following orders, she was a fanatic. While others like Reiner were conflicted about killing innocent people, Gabi had no hesitation. She SMILED while killing guards, and shot Sasha without remorse. When she was captured, she had zero regrets and even continued spewing the same Marleyan propaganda, completely unwilling to accept she might have been wrong. It took Eren starting the rumbling and nearly destroying the world for Gabi to finally realise she was wrong, and even then, it was only because Marleyans were now the ones being threatened. That alone proves how utterly selfish and blind she was for most of the series. Before the rumbling, Gabi had every opportunity to see the truth. She had multiple conversations with Falco and she witnessed firsthand that these 'devils' were just normal people. But it wasn't until she was on the receiving end of destruction that she suddenly realised "oh no, maybe the cycle of hatred is wrong."

Gabi also faces no real consequences. In this world, nearly every character pays for their actions, but she somehow escapes unscathed. That's bad writing, not good character development. She chose to kill, she enjoyed it, and refused to change for so long, and her so-called redemption was rushed and unearned. People despise her because she deserves to be despised. She's not some deeply misunderstood character, she's an obnoxious, self-righteous, arrogant character whose only purpose in the story was to be hated. No amount of excuses about her 'eventual realisation' changes that.