How was Gabriel so physically capable despite being locked inside Madison's own head for the better part of 30 years? Well if he's capable of putting her into a Lotus-Eater Machine, it's possible he learned how to do it because he did it to himself. Imagine a Matrix-style blank room where he could learn what he could do without actually doing it in the real world. By the time he's unleashed, he's turned himself into a killing machine.
At the start of the movie, he was already shown to be exceptionally strong. Also, it was stated he bided his time to regain his strength, and "ate" Madison's fetuses in-utero to absorb their strength as well.
Dr Weaver referred to Gabriel as a cancer to be removed. How does cancer manifest in a body? A malignant tumor.
A blow to the back of the head, like the one that awakens Gabriel, can actually be fatal. With how serious Madison's initial injury appears to be, and how malleable the back of of her head becomes... there's a very good chance that she's only alive because the back of her brain isn't technically hers.
