In my opinion Garp will survive this and if the storyline at some point needs him to die I find it more proper that it would happen in the future near the end and by the hands of the WG ( imu, 5elders, holy knights idk), in an open confrontation/war. It would really be a turnover for also the Marines and the world when their hero, a legendary marine (and not a pirate this time around) gets killed by the organization that they protect and believe that promotes justice. It will be a revolution, make the truth more credible. Lets not forget that Garp is also a D, and all D have defied WG at some point, even marines (Saul). Also why would he be so hyped in the entire manga, be such an important person to the main character and have him die like this. It would have no sense. Luffy Is not shown to be a revenge driven character.