There is a nice writeup about this case showing that the case doesn't deliver any evidence to demonstrate the view that personality has dependence on the brain.
Here a couple of quoted material from it
1850, two years after the accident, a Harvard professor of surgery stated that he was "completely recovered in body and mind", making no mention of any personality change.
In subsequent accounts by other writers Harlow's later testimony was embellished. Gage was now said to have become a drunkard and a boastful exhibitionist, as well as suffering an absolute lack of foresight - all unmentioned by Harlow. In fact most of what has been said about Gage subsequent to his physical recovery, Macmillan says bluntly, is "fable".
As we see then the case that many materialists use to bolster their case up is a mere fabrication. Sure there was some impact on his personality but not to the point where materialists proclaim it is.
You can read the rest of it here