I doubt he's dead, it just seems like things were rushed so they had to do what they did. I just want to know if Dalmasca's liberation will be for the Relic Weapon quest line or will it be a new plot point for 6.1+. That said ...

We also find out in another field record that Princess Ashe is still alive and she had an elaborate fake death that only a select few people knew about. It was intentional that people loyal to her think she's really dead so that the ruse can perpetuate itself. With Ashe faking her own, I just don't see Gabranth truly dying either. But why would he fake his death? To give Dalmasca false hope and distraction while he's out doing something