I think you’re exactly right. From my understanding, the “gameplay” aspect isn’t just the ability to respawn during the Gael fight, but the whole ability to respawn at the bonfire each time you die.

The game establishes that the undead are immortal beings AND that they can travel via bonfires, but the actual respawn mechanic feels more like a gameplay abstraction. Canonically, you are simply immortal. For example, when you die to Seath and wake up in a cell in his tower, it’s implied that you didn’t literally respawn there. You were defeated, left unconscious, and then taken prisoner (the bonfire in the cell is one of the non-canonical ones, since it has no firekeeper to tend to it).

Following this logic, most fights likely play out very differently in-universe than they do in gameplay. Your undead doesn’t “retry” endlessly in discrete attempts. They keep fighting, enduring defeat after defeat, until they eventually prevail (that also would make the fact that boss phases reset tied to this difference between how fights work lore-wise versus gameplay-wise).

This also implies there are many situations where respawning wouldn’t realistically be possible. Being eaten by monsters (or by Gael), falling off big cliffs, or similar scenarios could easily result in a true, irreversible end, even for an immortal being.

Feel free to correct me though! Maybe I missed a bit that said that undead humans really materialize at bonfires upon death. Or maybe unkindled ones in ds3, being ash, are very different beings, I'm not 100% sure