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Try a very thin piece of metal with some double-sided tape placed near the edge, and gently slide in and press down on disk. If you're real lucky it should come out.
 
To be fair, it probably doesn't state anywhere (except maybe in the manual) that it can't accept "Gamecube" sized discs. This would be possible if it was tray-loading and not slot-loading however.

But yeah, I'd just turn it sideways and try to shake it out.
 
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