Trepanning, or as it's now called, craniotomy, is where a section of the skull is removed / bored through. It's mostly done for cerebral edema where there's pressure inside the skull and on the whole brain (it can even fatally herniate the brainstem which means shoving it out through the bottom of the skull like a play dough extruder).

It's wild to think that there was actually a reason ancient cultures did it. They way overused it and for the wrong things during certain time periods and it was horrifying that they were doing it without anesthetic, but I've also heard that it results in a basically instant return to orientation. So the few patients it would've worked on would have gone from deliriously speaking in tongues (I know it's not any real language but that kind of confusion does at face value sound like something that would require an exorcism) and would suddenly just... wake up. Possibly with a spray of puss out of the wound.

There's a lot of old timey medical stuff we still do, it's just now we do it with anesthetic and sterilization. Medically sterile maggots are used to clear out dead and infected wound tissue and some surgeons who work on structures with delicate vasculature like hands will use leeches to prevent swelling from blocking off bloodflow to the area while it heals. I've spent most of my career working at places that do electroconvulsive therapy (again, under anesthesia) for severe treatment resistant depression and catatonia (like so bad they can't move or eat and need to be turned, cleaned and fed with a tube), and one time I worked with a patient who had had a frontal lobectomy (used to be called a lobotomy) for a severe seizure disorder that wouldn't respond to medication.

Anyway Gage's case was more on the subject of localized trauma and what injuries to the brain a human can survive. In particular it began our understanding that frontal brain injuries are usually much more survivable than ones to the rear, but that they can effect personality and in particular emotional and impulse control.