I personally don't believe Rauru was intended to be KG's secret identity for some reasons.
-Personality-wise they seem to have little in common. Rauru is completely to the point and no-nonsense, while KG is stated to be lighthearted. I know it can be taken as a pun of sorts, but sometimes things are as they are.
-KG never believed in the Hero of Time, however Rauru's entire purpose appears to be awaiting the same hero.
-KG speaks of it 'not being his time anymore', seems to me he's saying he's done all he can for Link. But this obviously isn't the case for Rauru, who Link needs to topple Ganon.
-Can any of you truly see Rauru saying 'Wait up, pal?'

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Memories of the world
before the fall remain...
But the light of dawn doth never change,
nor e'er the hearts of men...