I think Sunsweet's post was good.
... I'd say 'no' ... Gackt is still going strong, and Miyavi-sama and Gackt have different types of fanbase (although I'm not saying those who like Gackt can't like Miyavi or other way around) In Japan, Miyavi attracts girls who are more into rock, (used to be vk, but he changed), whereas Gackt has managed to attract an a lot more varied crowd of fans - from the 50 year old mothers, to the little, tv-game-playing guys, to the 'normal, common' fangirls.
Or that's how I see it. Gackt is bigger.
Although, I agree with Lady Blue: Recently, Miyavi-sama has become really popular everywhere on the internet - he's all they're talking about in all the forums. It's great! *dances*