To unlock the Gold Catfish, you must first find the Servants Ring located between the waterfall and elsewhere in Twilight Temple. When you find the Servants Ring, GG will appraise it before you have a chance to call Nancy, and will tell you it is worthless. Next, return to the Amazon where you found the Giant Catfish. A cinematic will play, and at the end you will have a chance to give the fish your ring. Choose "Yes". Do some exploring or something else, then return to the boat. On your next dive, you return to the catfish and another cinematic will reveal that the catfish is now a Gold Catfish.