It's really not even the Gary Stu. Any romance plot like this is naturally going to be a little power fantasy. The problem is the gosh darn everyman trope that equates to the main character having no character. Just being a bland self insert for the degenerates reading (and I have to say us degenerates would rather a well written character with a complex personality to root for). Often times even the love interests end up with one not personalities or their entire character being that they like the main character.
Any MC that attracts the most popular girls in school despite themselves not being popular is obviously a power fantasy stretch and at least I'm okay with that. That's what I'm getting into when I read the story and I know that. Most stories period have a bit of power fantasy and the plot pushing things along. I mean really what major thing did this MC do to "earn" hanging out with the most popular girls aside from being himself? Where it's to be awful is when "being himself" turns into "being generic vanilla man". It's those awful stories where the MC does a single nice thing for what the plot says is the most beautiful girl in the universe, and she instantly falls for him because no one has ever been nice to her apparently. And then throughout the story the main character doesn't really show that they have any character. They're "just kinda nice I guess" the character.