As I read the manga, I can't help but wondering where our favorite teacher Onizuka get his inhuman strength from? He's survived: falling from the school top floor, being shot at point blank, being smacked in the head, and many more. Is it because of his glory days as a motorbike gangster? Or is it because of his fighting spirit and will alone? Without his strength, the story would have ended much differently.
Onizuka can be considered a strongman. This is not the "strength athlete" variant of the strongman, though he might qualify, but more of the natural type. As detailed here, a strongman is not necessarily someone who actively participates in bodybuilding. It can also be someone of perfect symmetry or size, or those who have explosive power and powerful grip strength.
In Onizuka's case, while not really huge, is a former biker who has devoted (as of timeline of GTO) almost half of his life into fighting. His experience is almost akin to a full-time MMA welterweight or middleweight fighter in that he fights and gets exposed to brutal physical activity almost every day. This makes him, at the very least, much stronger than a good percentage of the general population.
While obviously his feats are a bit over the top in that he survives them almost unscathed (this obviously needs some suspension of disbelief), humanity has shown over the top endurance and strength feats as well. Falling and point blank shots to the head have been overcome by some, but the resultant damage of course has been of varying degrees.