Well, first, while I can't say for 100%, I don't think he is saying she was a Gyaru, just that Gayru as Japanese and tan their skin dark. So this was in answer to your assertion that, "no Japanese will ever turn that dark from sun tanning unless she consciously chooses to do a reverse Michael Jackson and spray tan herself to look like....that...." Thus, his response was likely that in the same way Gyaru do it, maybe she is doing it on purpose for some reason.
Second, in the same way not all Goth girls show all the features of, or embrace every aspect of, "Goth culture," Hana might just like that one aspect of Gyaru culture, the tanning. Sub-culture gatekeeping isn't usually effective, especially when done by people who aren't even part of the sub-culture. So the other guy might have brought it up in that respect as well. Hana is American, so her embracing a part of one part of a Japanese sub-culture isn't farfetched.
And I'm not saying that is why she is so dark, just that this is one or two possible reasons for it and I think was the point that
Black Orchid was making.