Literally ONE TIME. If people are actually complaining about more than just the one, I honestly don't know what to say to them. A few other characters CAN die/be removed/corrupted (Lizards, Feliar, Vicoria, Groth, etc), but there's routes or options you can do to prevent that from happening. Gunnar is the only unavoidable death, no matter what you do, to my immediate knowledge.
I'd be more annoyed at the fact that there's so many one-time scenes and all the save files you need to make to re-experience them.
That's the point: There's only ONE death that 'counts'.
For a story / narrative, that makes it impactful and meaningful, also makes the 'perfect' route more appealing.
Many great games / stories have memorable martyrs, satisfying and rewarding 'hidden route'.
But as such push back some fans do, It's very unlikely Hyao will touch any things remotely close ever again.
Also for other NPCs that can be removed from the game, it either goes nowhere or doesn't have any impact.
That's the main problem with this game's 'choices'.
And the simplest solution is just as I mentioned: No need to elaborate each scenario, just let them be variables for the 'perfect route'.
In a perfect world it'd be great if Hyao can expand upon each option, but that's impossible for solo indie devs.
As for those 'one-time' scenes and lack of 'event gallery', That's a quality of life refinement. Maybe Hyao will start to think about it after he sort things out, when the game is nearly done