Gackt is Squall Leonhart

Gackt claimed in a television program that Squall Leonhart of Squaresoft's role-playing game 

Final Fantasy VIII was modeled after him, and referred to Squall as "Gackt #2." There appears to be some facial and hair-style similarities (see above) as well as costume similarities (see below).

The left image of Gackt is from Malice Mizer's "Cyberland" period. Gackt was first seen wearing this costume in the CD booklet that came with the merveilles album, which was first released March 18, 1998. Gackt also appeared wearing this costume in Malice Mizer's "Illuminati" music video and during parts of Malice Mizer's "merveilles ~Shuuen to Kisu~" 1998 live tour. (On the "merveilles ~Shuuen to Kisu~ l'espace" live concert video, released October 28, 1998, he can be seen wearing the "Cyberland" costume during the performances of "Brise", "Ju te veux", and "au revoir".) The photo of Gackt on the right is more recent (it was taken after Gackt already left Malice Mizer in 2000). To compare dates, the first images of Squall were first released by Squaresoft in late (roughly fourth quarter) 1998.