Hi! I couldn't make a proper tag implication request because of the current relationship between gyaru and kogal tags and gyaru not having a wiki article. I think kogal should imply gyaru and not the other way around.
Kogal is a substyle of broader gyaru (or gal) fashion involving schoolgirls, and not all gyaru are kogals. I would even say that most real life gyaru are not kogals. So therefore although all kogals are gyaru, not all gyaru are kogals.
Many submissions currently tagged under "kogal" feature subjects that are not school aged and aren't wearing a kogal type of fashion (typically featuring gyaru-ified school uniforms, etc). Some examples are post #2588619, post #2570643, and basically all of the posts involving Plumeria. These subjects, at least in my opinion, should be tagged only with gyaru or gal, not with kogal.
I've had a lot of years of experience with wearing and tracking gyaru fashion, so I hope this request is accepted! Thanks.