I’d guess that he gave up something to free Alice from the tower and that something made him golden again.
The problem with that is that Alice ran up to Rumple the moment he stepped foot into the New!EF. In other words, she was free from the tower already, unless it was only temporary freedom??? But why would it be Rumple who could free Alice from the tower as opposed to either her father, Hook, or her true love, Robyn?
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"He was a lot of things to me" "The only conclusion was love"