If every student had a unique gymwear outfit I would be advocating for outfit tags for every character. No matter how generic (but recognisable) they might be, the characters wear the outfit officially and so artists draw it. At this moment the correct way to tag these costumes is gymwear + official alternate costume. If we acknowledge the presence of these outfits anyway, why aren't we just tagging them?
If Hina wears the Abydos Sportswear outfit, then it's: gymwear + cosplay??? Even though users recognise the outfit and the artist is drawing the specific outfit with intent, there's no way to indicate it.
Gehenna Academy Sportswear doesn't exist at the moment, so the alternative post #7323433 ended up resorting to was tagging it as a cosplay of Haruna. Others have created tags like Izumi (Track) (Blue Archive) for lack of a gentag.
The (Track) characters just show we're already identifying and tagging these outfits anyway without issue. Hoshino has over a hundred posts of the Abydos Sportswear outfit by herself. Hundreds more posts have that same outfit on other characters. That many posts of artists trying to draw the same thing should be grouped under a single tag.