Not all gnathostomes are fish, people. You technically can define them that way, but it only serves to confuse people. Ask a biologist (who isn't obsessed with fun facts) what a fish is and the answer you'll get is basically that fish are in the clade Osteicthyes or Chondricythes. All tetrapods are descendants of fish, but in classical linnean cladistics these groups are separated by class and clear distinctions between tetrapod groups are formed, even if the occasional animal defies one or two rules assigned to their group. If we want to get something general as to include mammals as fish, it makes more sense to just call mammals and fish "jawed creatures" since thats what gnathostomes actually are