Japan: For a country being known as favoring white skin...?

Question:

Why is Gyaru popular? I'm very curious! I can't find a lot of information as to why tanning and Gyrau is popular, considering that bleaching the skin in order to have whiter skin is popular too.

Answer:

Ganguro is outdated and not popular now.
I think the "ganguro" look is a form of fashion rebellion or fashion anarchism of the youth way back in the 90s. As you said, many are obsessed with white and fair skin so some girls went for the different look and started tanning their skin (really tan) and putting heavy make ups.
For the regular "gyaru" look, these girls are trying to achieve the "California beach girl" look. With medium tan skin, blonde wavy hair, bronze make up and summer sexy clothes. Gyaru is still quite common now.

But you should know, ganguro is only a type of a gyaru.