How can one survive a gunshot wound to the head?

Trauma centers: These centers save lives! it is amazing, but a gunshot wound to brain can result in meaningful survival. First, the trauma center prevents secondary injury (e.g. Low oxygen levels in brain). The rest is some degree of luck (what part of brain is damaged, ballistics, etc).

Luck: It's simply amazing to see how resilient life is, while at the same time being so fragile. But simply stated, the ability to survive, and even recover from a gunshot wound to the head is dependent first on how much brain injury is sustained, what part is injured, and how quickly surgical intervention is performed. This is exemplified in the case of congresswoman gabrielle giffords.

Depends what it hits: If the bullet or force of the blast does not hit any of the important life maintaining structures AND swelling, bleeding does not crush the brain. .. then one can survive a GSW to head. But most don't.