Throughout his career GACKT has covered many artists. Here's a selection of them in no particular order...
(Part I where other artists cover GACKT, can be found
here)
Mr Children - 抱きしめたい (Dakishimetai)
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西城秀樹 (Hideki Saijou) - 傷だらけのローラ (Kizudarake no Lola)
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Bonus live I love this so much
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John Lennon - Love
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T-Bolan - 離したくはない (Hanashitaku wa nai)
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Gundam - 哀戦士 (Ai Senshi)
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Bonus Live
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Gundam - めぐりあい (Megurai)
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レミオロメン (Remioromen) - 粉雪 (Konayuki) with T.M. Revolution
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Bonus
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中島美嘉 (Mika Nakashima) - 雪の華 (Yuki no Hana)
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郷ひろみ (Hiromi Go) - 逢いたくてしかたない (Aitakute Shikatanai)
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Traditional - Silent Night with Ayumi Hamasaki
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Bonus solo
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B'z - いつかのメリークリスマス (Itsuka no Merry Christmas) with Ayumi Hamasaki
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Bonus with Namie Amuro
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Stevie Wonder - Feel the Fire
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Bonus CM (couldn't resist... XD)
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There are also other artists he has covered as part of the Camui Gakuen tours, but unfortunately no real footage exists of these shows. Artists covered so far at Gakuen include Glay, BUCK-TICK, LUNA SEA, The Checkers, L'Arc~en~ciel, AKB48, SMAP, X Japan (GACKT on drums, Chacha on vocals), The Bluehearts (GACKT on drums, Jun-ji on vocals) and more.
I decided not to include Kagero as although it was originally a solo Chacha song, GACKT wrote the lyrics and sang the original vocal.
Of all of these, his version of Konayuki is my favourite. He sang it at Gakuensai in 2010 (solo, of course) and it was amazing. I wish that there was a recorded version of it - either studio or live.
As before, if you have any more please add them in the comments!