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Though there was an official Mario Pikachu, these kind of gold-back cards are fan reproductions (fake). Kind of unusual that they didn't make the front gold as well, though.
I think the gold back might be a sleeve on the card??? It's really hard to tell with that picture. Have you tried taking it out of the case?
If the card actually has a gold back than it's a fan reproduction like Icycatelf stated. I've not read anything about a legit Mario Pikachu being printed with anything other than the standard card backs.
Exactly what I was thinking. I feel like I see a crease to a sleeve in the hard case but it's not the best quality pic so it's hard to tell. If it's indeed a sleeve than you most likely have an authentic piece.