Gage was a railroad foreman who in 1848 had an iron rod completely driven through his brain in an unfortunate, accidental explosion. The puncture destroyed a significant portion of his left frontal lobe, though miraculously (especially considering the medical capabilities of mid-1800s physicians) he survived, thanks in large part to the treatment of J.M. Harlow, one of the few doctors of the time to have any experience treating brain injuries. But you don't need to be a neurosurgeon to guess that having an iron rod impale a person straight through the brain might have lasting complications.
After the accident, Gage became a minor celebrity just for surviving and retaining functionality. He could still speak, move, and seemed to exist as Phineas Gage. Soon, however, Dr. Harlow claimed people close to Phineas said he was "no longer Gage," and went on to say that he became difficult to manage, profane, irritable, and impulsive. Of course, you would probably be all those things if you'd had an iron rod shot through your brain, but at the time, it was groundbreaking to associate specific areas of the brain itself with personality.