It's interesting, because when I first started reading 'Fairy Tail' fanfiction years back, Gajeel and Levy stories were everywhere, but they were mostly these super tropey 'bad boy softens for the good girl' things. You still see that, obviously, but I feel like the evolution has gone a bit deeper lately. The Gajeel that gets written now often has to grapple with the weight of what he did to her guild, to her specifically, beyond just a surface-level apology. It's not just 'I'm sorry' and then instant romance.
I've read a few really solid fics where the author uses Levy's love of books and words as a literal tool for his redemption. Like, she teaches him to read, not just as a cute bonding moment, but because his illiteracy is tied to his history with Metalicana and his self-worth. Him learning to decipher runes or write his own name becomes this huge symbolic act of reclaiming his identity outside of being a weapon. The evolution there is from a weaponized tool to a person who can define himself.
Another shift I've noticed is in how his relationship with the rest of the guild is handled. Early fic often had him as this lone wolf tolerated only for Levy's sake. Now, you get more stories exploring his awkward, gruff attempts at being a proper guildmate—trying to pay for his damages, getting roped into team missions with Jet and Drayer, that kind of thing. His evolution is as much about integrating into the community Levy represents as it is about the two of them as a couple. It feels more holistic.
Honestly, the weakest fics are the ones where his evolution is just a costume change—he's still a snarling jerk, but now he's Levy's snarling jerk. The best ones make you believe in the slow, difficult change of a man rebuilding his entire moral framework, with Levy as the catalyst and guide, but not the sole reason for it.