Both of his big injuries, the shoulder injury in 16 and his ankle injury this year, happened because he was out of the pocket running around trying to make a play happen. I don't know if he is injury prone but one thing he isn't, and this is obvious at this point, is he isn't Fran Tarkenton
If you ask me what the reason is, I'd say his unique career track is a factor. His mind is way, way ahead of his body right now. He sees that a play is possible but hasn't learned what his body can or can't do at the NFL level, and he overextends himself based on what he can see mentally and can't pay the physical price. If he'd played, he'd know what his body could or couldn't take but because he hasn't played, he's just learning that now.
Staying in the pocket is absolutely a factor here, and being able to stay in the pocket requires a good O-line. If I was coaching Jimmy G, that's how I'd try to fix it, I'd rein his scrambling impulse in, force him to stay in the pocket and put the impetus on the offensive line to keep him safe. He understudied Brady, he ought to know everything there is to know about dominating from the pocket. Leave him there, make him stand in the pocket and let him work.