Unbelievable GT (Giant Trevally) Attack on Bait. The Giant Trevally (GT) is known as one of the most powerful fish in the sea. They are a highly sought after sport fish, and to catch a big GT (they get well over 100 pounds) on a fly rod is usually on the bucket list of most avid fly anglers. These GT are basically built like NFL running backsâ¦.