"G" isn't Gackt, Gackt is the artist who did DC's theme song. He also did the acting for the secret. "G" is Genesis, who IS in Crisis Core.
Spoiler:
"There is no time for slumber.
There is still much work to do,
My Brother."
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Theory: Genesis and Sephiroth are brothers, born of Jenova and Lucrecia. Thus, the opposing wings.