More specifcally, what sort of medical interventions did he receive and can any of them be attributed to his seemingly miraculous survival? Thanks in advance.
Luck. Pure simple luck.
Gage was in great physical shape, first and foremost. His tamping rod (with which he packed the blasting powder that caused the accident) was a straight tapered cylinder with a point on the end, which Gage had custom made. So it passed through his skull relatively cleanly. The rod went through the least important part of the brain (the frontal lobe is relatively unimportant from a survival standpoint) and did not open up any major vessels or hit the sagital sinus (which would have drained his cerebrospinal fluid, causing rapid death).
He survived a serious infection of the orbit of the eye and the wound channel, which was drained by a physician who had been taught about tenpaning and had seen a famous surgery in medical school demonstrating the usefulness of drilling a hole in the skull. The physician, John Harlow also had the uncommon good sense to leave the wound open, losely dressed and he put on silver nitrate (an early effective bateriocidal). Harlow also didn't poke around too much for skull fragments or try to pack the wound with gauze.
Harlow himself says, "I can only say ... with good old Ambroise Paré, I dressed him, God healed him"
Macmillan, Malcolm B. (2000). An Odd Kind of Fame: Stories of Phineas Gage. MIT Press. ISBN 978-0-262-13363-0.