In the anime Hajimete no Gal (aka My First Girlfriend is a Gal), it's made abundantly clear in the first episode that "gal" is not just another feminine pronoun. Based on the 2 "gals" in the show, and compared to the other girls in the show, it seems there's a marked difference in style, personality, and appearance to be labeled a "gal".
What is a "gal" in anime or Japanese culture?
"Gal" is the English transliteration for gyaru, which is a fashion trend in Japanese culture. This fashion trend includes things like:
Effectively, the "gals" in My First Girlfriend is a Gal are following the gyaru fashion trend.
Can't comment yet so I'll add an answer.
ギャル (gyaru) is actually the way they imported the English word "girl".
Then they imported the English slang "gal" on top of that as a back-Romaji-ized name for the current fashion.
(In addition to what Makoto said.)
If you can read the Japanese, here's a dictionary entry, but it doesn't help much:
https://dictionary.goo.ne.jp/srch/en/ギャル/m0u/
Here's a wikipedia entry:
https://ja.wikipedia.org/wiki/ギャル
And here it is in English:
https://en.wikipedia.org/wiki/Gyaru
The Japanese is more complete and more interesting, of course.
There is also a bit of a deeper meaning and as to why this trend came about. It goes way back to the World War 2 and a bit before. After occupation to make money, girls would well prostitute themselves, so a tanned girl was also a sign that they were out on the corner a lot. In the rebellion days of youth, in the 80s, it was a way to rebel in Asia, so it became a popular trend.
This is why you also get that "gals" are sluts or whores. In the west though, tanning became a positive thing while in the east it became a negative thing. Also why in the anime and the manga of Hajimete no Gal, the ex calls her a "slut". Also, Hajimete no Gal directly translates to "first time gal".
