So basically, Gawr is Gura in the Myth's Bad Ending animation when her eyes change from black to white.
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yes, but only after her attack. I don't remember what game it was, but I do recall a last resort attack that happened when a character died. Also FF VIII had a move where one of the character could revive the whole party even if everyone was died, if I recall right. So, the way it would work is her HP would hit zero, she would change into Gawr mode get off one final attack (even if it's the enemy turn a counter if you will) then fall unconscious. At least that's what I was thinking.