I think it's really just about your own personal preferences and style of choice. I agree that some of them over do it and it just looks odd, but other girls I find are very good at the whole "gyaru-gal" thing.
I follow this girl in Finland, she has her own blog and seems to call herself a gyaru. She has a great sense of style and I love everything she wears and does with her hair/make-up/nails/etc. You can see for yourself here:
HauteKeikk.com
She wears contacts, fake eye lashes and extensions just like your typical gyaru in Japan, and still manages to make it look good (at least to me she does, I really don't see anything wrong with her style) even though she's a "westerner".
Personally I think it's alright if you adapt it to your own style and body. Some girls over do it, yes, but some western girls can often over do it with western styles and clothes also anyway...