[habelie]

Hmm...yes, I would say Gackt too.
Besides sales and influence...they have some similarities, and I can't really think of any other solo male.

1. Both of them are very good looking, have a similar sort of prettiness, and are types that leave a strong impressions.
2. Both of them are dramatic stage performers, though Gackt definitely tends to...move more.
3. They both have uncommon unique types of voices. Of course everyone has a unique voice, but their singing styles are more...recognisable? Ayumi has a very high-pitched voice (used to at least), while Gackt has a pretty low one. Instead of choosing a guy who sings in the higher range, I think it's more interesting to have a contrast? XD Kind of like (extreme?) representation of either sex.
4. Adding on to the previous point, they're kind of like the 'perfect' image for males and females. I mean, there was even this manga that pointed it out. Something like how no one could be as perfect as Ayumi and Gackt? Saw it on a thread a long time ago.
5. Lastly, I think that they both like to have control over matters. I'm not too sure about how Gackt works, but he gives me that impression of a perfectionist, much like Ayu.

And that ends my rant ^^;; Oh, and of course, they're very fashionable =D

Oops, I don't suppose I'm really answering the question.
In terms of music similarity...I think their lyrics for romantic songs. It's very hard to explain, but the details in them hold very deep sincere feelings. And also, the use of words, though a lot of Jpop songs use the same words, Ayu and Gackt's lyrics are more poignant, they're not as...playful. I can't find the right word...