Gulp Fish are apparently not reproducing in the wild. In my current game I had the misfortune of having a Pacu lay a Gulp FIsh egg. The egg hatched and, at about 10 years of age (about when you would expect the fish to lay an egg) the Gulp Fish suddenly burst and turned into meat and a bottle of water. Enclosed is a save file with the camera on the Gulp Fish right before it self destructs.
Steps to Reproduce
This is random due to the chances of a Gulp Fish Egg being laid by a Pacu. However it might explain why this is the only time I've ever seen a wild Gulp Fish in the game.
This issue has not been confirmed by a developer yet.