Here's an explanation. Normally, if the Japanese uses an English word, I'll use that word in the translation if at all possible. In this case though, it's not possible since gal means two completely different things to English speakers and Japanese speakers. Please see the translators note on page 2 for an explanation. For a more thorough explanation, search gyaru on the web.
Thanks for the answer and I deeply appreciate and enjoy your work bringing yuri manga to us. However I simply don't agree to your point. You say it's just not possible to use the word "Gal" in this instance because the meaning is different in Japanese, but I don't buy that. You're already using a translator's note to explain the made-up word "gyaru". How is that any different than using a translator's note to explain the use of "Gal"? "*In this instance the English word 'gal' means..." etc etc. Not only are you more faithful to the text but you've also educated your readership, not all of whom will know that gyaru is a transliteration of gal.
I'm not trying to badger you over this, and we will probably agree to disagree, but I just wanted you to see this issue from this reader's point of view. If you still feel your way is more appropriate, then I will respect that and back off.