In my honest opinion, I don't think he is dead. I shall create a story of what happened in Johnny's point of view:
"Johnny, we're about to jump." Shaundi stated from the radio. I had the door locked so those syndicate thugs couldn't get in, but the door was starting to give way for the black-and-red gang. It seemed like only moments ago we were being held at gunpoint by those bastards. "Right on, I'll see you in stil-..." As I replied the door collapsed and the gang began shooting at me, then one of them shot the communicator and it went bust. I managed to take most of them out while only being shot twice, then i heard more coming and I lost all my fight then. Aisha would have been ashamed of what i've become; a corporate-whore like what happened to her, but then she faked her own death. I knew I could do the same and lay-low until I could get the saints back to the streets where it all started. I quickly traded clothes with one of the dead gang-members on the floor and stole a parachute from the storage. I couldn't give up the shades, so I kept them and everyone would assume they got lost in the wreckage. I could see the boss falling through the air and Shaundi was doing the same, but without a parachute. I staggered through the plane until i reached the back, where the boss and Shaundi jumped. The gang-members were jumping down as well, so I could easily escape. I jumped down and fell through the air, feeling at peace with everything. I hadn't felt like that since Aish was alive, I was completely bummed out at that point and needed a vacation. As I slowly madde my way towards Steelport, I decided not to go back to the boss just yet so I could have a break, at least for a while. I pulled at my parachute and the huge, grey sheet flew out of the bag, leaving me to float to the ground. Steelport was my perfect place; sex, drugs and crime. I found an empty apartment block and made my plan. The saints were going back to the way they were supposed to be, boss or no boss.
I know it kind of sucks and seems very unrealistic, but Gat was with the saints for years and probably deserved a break. It would explain "Gat's" body and zombie Gat. Some may say he should have died because gang-members die, but not every game NEEDS to have a saint's death.