Ok so. I have a theory of why Gabriel is not dead, based mostly on the plot of hammer of Gods.
First of all, I shall start with the question whether Gabriel thought he was going to die or not. Before confronting his brother, we all know he had made a “if you watch this, I’m dead” video tape and gave it to the Winchester brothers. It’s clearly that the tape was destined to THEM and only, so, that could only mean that he already knew he would meet Sam and Dean sometimes and that he has to give them such a tape. If you watch closely, you can see perfectly that the two appearances, the one from the tape and Gabriel in the episode, are different : other haircut, other outfit. Yes, you could say that he is the trickster and he could have pulled out a trick to manage to do that, but most unlikely… He made the tape long time before. So, why would you ever make a tape like that unless you have some suspicions concerning your life and whether you will die sooner or later?? He made the video to inform the brothers about a way how to stop the apocalypse, so he knew, or at least, the thought of him dying before Armageddon happens, before Lucifer being killed, crossed his mind.
Now, who or what could have been such an important threat towards his life in order to actually make a video tape? He is an archangel after all, you can only murder him by using an archangel blade. Two options we most likely have in this case : 1. someone who is very powerful and who could some way steal his blade, or 2. someone who already has one. Yet, if in the first case you could consider the Gods from the meeting, that he already knew he was going to join in, then remember the fact that he literally said he wouldn’t have been that stupid to let Kali steal his veritable blade, because that could have killed him.
So, the first option instantly falls. It’s someone who might have an archangel blade. There is no point in adding that the only threat is Lucifer. Gabriel knew he was going to fight his brother, he knew he might die, he knew he might get killed, he knew HE HAD TO MURDER HIS OWN SIBLING, he knew he might not be cunning enough to trick him after all. He gives the tape to Dean and Sammy right before talking to Luci, so he just knew way before that might even happening. He knew he might never see them again. He wanted to risk his own life to save the humankind by killing Lucifer WAY LONG BEFORE THE WINCHESTERS EVEN MENTIONED THAT POSSIBILITY. He had thought of sacrificing himself long before anyone else thought of that. That kind his soul was.
But now the problem arises! If he had indeed murdered Lucifer, the way he thought he would (also, from the look of surprise on his face it’s pretty clear he was quite shocked by his failure), then the apocalypse issue would have been stopped! But now that he died, the problem had to weigh down the brothers’ shoulders again. Gabriel’s death was useless!! He could have told Sam and Dean about the cage without getting killed. He could have solved everything in that way better. Yes, still, there WOULD have been the chance of him actually killing his brother and solving the issues faster and more comfortably, so he literally stood up for himself and humanity and risked everything he had just to make the Winchesters’ life easier… Still, the possibility of him dying doesn’t help them at all!! So what if he was sure he wasn’t going to die?? Why the heck would you ever join a confrontation where you might die, and your death would serve for nothing? I kind of feel that he mostly served himself without any cost to Lucifer. The battle was already lost. He was either going to kill his own brother, or most probably he was going to die, like he just knew… Yes, he wanted to finally solve his family problems and help the brothers, but Gabriel is not the type of loser. And he is the trickster. I sincerely believe he is not dead because he had no reasons to take that chance. He had no reasons to get in such a battle that would bring up his death for nothing. He had an escape plan. That’s for sure. Him fighting Lucifer is either a really brave, outstanding and relatively stupid plan, either a masterpiece trick. So, he’s either waaay too good and brave for this world, or he is alive.