no intention of reading this slop but i just wanted to say after reading the title that gyaru isnt a gender. 'gender-swapping' cant make you a gyaru, mc chose to be one all on his own or it wasnt gender-swapping
congratulations, you've missed the damn point.
The point is that this socially awkward doofus has been genderswapped into a young girl who has a lot of physical attributes, attire and cosmetics associated with Gyarus and is then transferred to a school full of em.
The mind remains the same because that's where the conflict and comedy comes from, the shy person getting absolutely immersed in a culture of very loud, flashy and extroverted people, adjusting and camouflaging as best they can is an act of adaptation, this kind of fish out of water trope is... pretty common in fiction bro.
Hell, if we wanted to delve deeper, we could infer that some amount of Gyaru knowledge subconsciously dwells in the protagonist's head without them noticing, since they don't seem to be breaking those long nails or jabbing them into things.
That is admittedly way more thought than this silly little story merits, but that just goes to show how YOU are getting way to uptight about a silly little story in the first place, unclench and relax.