This is a pretty revelatory for ANY kids' movie villain, even today: Gaston's NOT EVIL. He's a dick, he's often selfish, he's misogynistic as hell (as literally every dude of that era was) - but he's not evil. He's not trying to murder puppies to make a coat, he's not trying to poison a young girl because a mirror said she was pretty - all he really wants is to marry Belle, which (in his mind, at least) will give her a better life. He's misguided, but he's not evil.
He even genuinely tries to woo her - he never does anything truly villainous, like - say - kidnap her and hold her hostage in his dungeon. I mean, that would be a SERIOUSLY evil thing to do, right?
In the climax of the film, Gaston and his angry mob storm the Beast's castle with the aim of killing the Beast. We, as the audience, know that the Beast has changed and is a nice (but wounded) dude and he and Belle are totes in love. But from Gaston's point-of-view, the Beast is...a beast. And his house is filled with an assortment of witchcraft-esque LIVING FURNITURE. And - sure, Belle is saying everything's cool, but who knows? Maybe (as many have pointed out over the years) Belle has Stockholm Syndrome. Or maybe she's under a spell (after all, how is Gaston supposed to know what kind of magic is going on here?).
If you were Gaston and you knew the girl you had a crush had been held captive in a freaky castle owned by a Beast and staffed by living furniture, you would be right to fear for her safety - and considered pretty brave for daring to lead a charge against these kidnapping-happy abominations.
"Gaston" is a helluva villain song - partially because it's not REALLY a villain song. It's not Gaston boasting about his evil plots, explaining how he's tortured by his desires. It's joyous and uplifting, a legitimate drinking song. It's Gaston and his buddies having a good time - because that's who Gaston is.
It starts with his sidekick LeFou trying to cheer him up - this isn't a madman villain raving and ranting about his own personal glory. This is a friend trying to brighten up his buddy's day. And as it goes on, one thing is enormously clear: Gaston's got a lot of friends here. He's genuinely beloved by this community - hell, the only people who seem to NOT be super into Gaston are Maurice and Belle. Gaston's not someone who's terrorized the community or behaved monstrously - he's done a lot of good for this town, and they want to support him in his time of need.
You see, Gaston's a man of the people. Unlike princes who live up far away in their high castles, Gaston drinks with the common folk. He hunts game to keep the village stocked up with meat. And - at five dozen eggs EVERY MORNING - he's clearly invested in keeping the local economy humming along (especially whoever sells eggs).
Let's face it - Gaston is one handsome dude. He's got a killer jawline, good hair, good teeth (and this is 19th century France!), and is jacked as hell. He makes Prince Eric look like Michael Cera with a calcium deficiency. Basically everyone in town is splooshing like crazy over him EXCEPT Belle.
For pretty much EVERY Disney film up to Beauty and the Beast, it was very easy to tell who the bad guy is - if they're ugly, they're evil (think Cruella De Vil, Maleficent, Ursula, etc.), and if they're handsome/beautiful, they're good. But Gaston defies that trope entirely - being handsome AND the antagonist.
Gaston proved handsome people can do anything, even be the bad guy*.