It begins all the way from his point of origin in Rave, where he held essentially the same role as he does in Fairy Tail. Despite Mashima's love of him, it never felt like he put any too much thought into him, other than deciding to give him a nice hair cut. Even the names were very similar (Sieg Hart / Siegrain).
When it was finally Gerard's time to shine, he hadn't done anything to reverse my negative first impressions of him , and we were now dealing with two different versions of him. I wasn't a big fan of the Seigrain half, but we also got the variation of Gerard who was sitting at the top of the Tower of Paradise acting like a fairly commonplace antagonist. At this point we've got two Gerards; One I did not like, and one that was ok. I also liked his relationship with Erza at first. And had he died like we thought he had, he would've been a way cooler character to me, 'cuz frankly he would've went out in an awesome way.
But anyway... Surprise, surprise, the very next arc following Tower of Paradise climaxes with the big reveal of Mystogan, a really cool character up until this point, taking off his mask and revealing himself to be some sort of clone of a character I now double dislike. Of course, I'm not excited, but if Mystogan had actually done anything cool after revealing his face, I might have not cared and accepted him as "The Cool Gerard." He just ran off tailoring to Mashima's penchant for off screen nonsence.
Again, the very next arc, a huge plot point ends up being... Gerard! This time it's much, much worse. He comes back with one of the most lame cliches in writing - amnesia. Now I don't mind if something is cliche as long it entertains, but it didn't really. This essentially means he's got a new personality - and a terrible one at that. While I thought Gerard did do a couple of mildly cool things, his presence only further deteriorated Erza's character and he stepped in at the final moments to provide the essential deus ex machina Natsu needed to win. The Flame of Rebuke, something so integral to the final fight, literally come out of nowhere.
And to no one's surprise, the succeeding arc revolves around his clone again, who seems to have removed his mask for the final time. Seeing as how Mystgun is a different person, I've still got a little hope for his character. But no, even Mystgun's involvement becomes convoluted.
Would it have killed Mashima if he had at least hinted at the existence of that "prince" beforehand? Like Panther Lily or some other unhappy bloke in the king's army saying "if only the prince were still alive", or something like that. Unsurprisingly Mystgun still manages to combo break the ending of the arc by having his suicide be their saving grace. He opts for the Watchmen/Dark Knight/Lelouch-esque ending and fails to follow it through successfully anyway. Gazille, Wendy, and Natsu manage to bring it back through comedy thankfully.
In short, the hatred for Gerard is summed up as: Gerard is annoying. He shows up constantly, and never makes the story any better for it. Mashima tries to sell him as a martyr unsuccessfully. He's a recycled character design to begin with, and then he's constantly recycled within the story itself. He never does anything that anyone likes, cares about, or wants to see.