Itsuki is also used as a feminine name in Japanese. However, it's more common for males. So, I think that the "Gender" of this name should be changed from "Masculine" to "Masculine & Feminine". Also, according to the source below, "from the late 7th century to the 14th century, an unmarried princess of the Imperial Family, called "Saiō" (斎王) or itsuki no miko (斎皇女), served as the sacred priestess of Amaterasu at the Ise Shrine upon every new dynasty.[107]"Sources: https://en.m.wikipedia.org/wiki/Itsuki https://forebears.io/x/forenames/itsuki https://en.m.wikipedia.org/wiki/Amaterasu
Itsuki, used as the kanji "樹" is currently 4th place in Japan, in terms of kanji and Itsuki used as "いつき" is 33rd place in terms of sound. (source: http://5go.biz/sei/cgi/ninki1.htm).
This COULD technically be used on a girl as well with different kanji. It doesn't seem like a super-common name these days. I would use Itsuki on a girl using the kanji 生連基 and it probably never would have crossed my mind for a boy. That said, ki can be an ending for names of either gender... though certain kanji are more likely to be used on 1 gender or the other. However, I guess the kanji I have here could probably be used on a boy as well.
Sources: https://en.m.wikipedia.org/wiki/Itsuki
https://forebears.io/x/forenames/itsuki
https://en.m.wikipedia.org/wiki/Amaterasu