To be fair, Gilda's actions 'was' a lot more petty than Discord's, but honestly – Gilda was never as versatile or fun to watch as he was. He was always put on a much higher level of villainy, so the stakes in forgiveness would be equally as high. Gilda, on the other hoof, she was just kind of a rude bully who didn't like ponies in her way, whereas Discord was acting more for the sake of his own entertainment and desire to rule than anything else.

When you think about it, the Elements of Harmony would reasonably count as a target for him to mistreat right off the bat. He lost 1,000 years imprisoned in stone because of them, so the ones currently bearing them shouldn't get any easy treatment… Especially when they're defending the ponies who imprisoned him in the first place. Obviously that's not a just reason to brainwash anypony, but at least there's some motivation we might sympathize with a little.

With Gilda, though, she was just a grump who didn't really have a good reason to go and scare Fluttershy like that. It was pretty uncalled for, and she lacked any reasoning for us to like her in the first place.

See, it's funny because Gilda yelled at Fluttershy that one time and Discord only tried to destroy her self-esteem through verbal abuse and then mind-raped her!

So take that, Gilda!